The shoplifting statute creates immunity for shopkeepers under certain circumstances:
Quote Quoting Wisconsin Statutes, Sec. 943.50 Retail theft.
(1) In this section:
(a) "Merchant" includes any "merchant" as defined in s. 402.104 (3) or any innkeeper, motelkeeper or hotelkeeper.

(ar) "Theft detection device" means any tag or other device that is used to prevent or detect theft and that is attached to merchandise held for resale by a merchant or to property of a merchant.

(as) "Theft detection device remover" means any tool or device used, designed for use or primarily intended for use in removing a theft detection device from merchandise held for resale by a merchant or property of a merchant.

(at) "Theft detection shielding device" means any laminated or coated bag or device designed to shield merchandise held for resale by a merchant or property of a merchant from being detected by an electronic or magnetic theft alarm sensor.

(b) "Value of merchandise" means:1. For property of the merchant, the value of the property; or

2. For merchandise held for resale, the merchant's stated price of the merchandise or, in the event of altering, transferring or removing a price marking or causing a cash register or other sales device to reflect less than the merchant's stated price, the difference between the merchant's stated price of the merchandise and the altered price.
(1m) A person may be penalized as provided in sub. (4) if he or she does any of the following without the merchant's consent and with intent to deprive the merchant permanently of possession or the full purchase price of the merchandise or property:
(a) Intentionally alters indicia of price or value of merchandise held for resale by a merchant or property of a merchant.

(b) Intentionally takes and carries away merchandise held for resale by a merchant or property of a merchant.

(c) Intentionally transfers merchandise held for resale by a merchant or property of a merchant.

(d) Intentionally conceals merchandise held for resale by a merchant or property of a merchant.

(e) Intentionally retains possession of merchandise held for resale by a merchant or property of a merchant.

(f) While anywhere in the merchant's store, intentionally removes a theft detection device from merchandise held for resale by a merchant or property of a merchant.

(g) Uses, or possesses with intent to use, a theft detection shielding device to shield merchandise held for resale by a merchant or property of merchant from being detected by an electronic or magnetic theft alarm sensor.

(h) Uses, or possesses with intent to use, a theft detection device remover to remove a theft detection device from merchandise held for resale by a merchant or property of a merchant.
(3) A merchant, a merchant's adult employee or a merchant's security agent who has reasonable cause for believing that a person has violated this section in his or her presence may detain the person in a reasonable manner for a reasonable length of time to deliver the person to a peace officer, or to his or her parent or guardian in the case of a minor. The detained person must be promptly informed of the purpose for the detention and be permitted to make phone calls, but he or she shall not be interrogated or searched against his or her will before the arrival of a peace officer who may conduct a lawful interrogation of the accused person. The merchant, merchant's adult employee or merchant's security agent may release the detained person before the arrival of a peace officer or parent or guardian. Any merchant, merchant's adult employee or merchant's security agent who acts in good faith in any act authorized under this section is immune from civil or criminal liability for those acts.

(3m)
(a) In any action or proceeding for violation of this section, duly identified and authenticated photographs of merchandise which was the subject of the violation may be used as evidence in lieu of producing the merchandise.

(b) A merchant or merchant's adult employee is privileged to defend property as prescribed in s. 939.49.
(4) Whoever violates this section is guilty of:
(a) A Class A misdemeanor, if the value of the merchandise does not exceed $2,500.

(bf) A Class I felony, if the value of the merchandise exceeds $2,500 but does not exceed $5,000.

(bm) A Class H felony, if the value of the merchandise exceeds $5,000 but does not exceed $10,000.

(c) A Class G felony, if the value of the merchandise exceeds $10,000.
(5)
(a) In addition to the other penalties provided for violation of this section, a judge may order a violator to pay restitution under s. 973.20.

(b) In actions concerning violations of ordinances in conformity with this section, a judge may order a violator to make restitution under s. 800.093.

(c) If the court orders restitution under pars. (a) and (b), any amount of restitution paid to the victim under one of those paragraphs reduces the amount the violator must pay in restitution to that victim under the other paragraph.
As I see it that first provision can be read in two ways: The first, which I think is the proper way, is that a merchant can choose whether or not to prosecute a minor and if he chooses to do so may detain a minor "for a reasonable length of time to deliver the [minor] to a peace officer or, if not, can contact the minor's parents and hold the minor for a reasonable time until he can be delivered to his parents. A strained reading of the statute would be that the merchant can only detain a minor until it's possible to turn the minor over to the police or a parent or guardian; I'm skeptical that a court would accept such an interpretation, but you and your lawyer can check the case law and consider trying to convince a judge to uphold such an interpretation.

If your son had his phone and was able to make and receive calls, it can't be argued that he was denied access to a phone. If the police weren't summoned within a reasonable amount of time, you may be able to argue that the duration of the detention was unreasonable such that immunity should not apply. If your son consented to the search, he consented to the search. If the store was closed when you arrived, it's not a big surprise that you spent some time waiting outside.

Keep in mind that if you do try to bring a claim, you would have to prove damages. How much time are you alleging passed between when your son was caught shoplifting and when the police were called? Is it possible that the store will argue that they had planned to release him to you, but that your conduct at the store convinced them that they should instead summon the police? (Don't take offense at that question - you would be shocked at how some parents behave under this type of circumstance, ranging from explosive anger at store employees to threatening to beat their child right then and there.)