1) I wouldn't do anything without first involving your company's lawyer and insurance carrier.
2) It will obviously be beneficial - actually probably a CYA necessity - that you are able to document the suspicion of theft and the notification of the meeting (prior to your knowledge of his medical status?), before you take any employment action.
3) Obviously you don't want to be paying him salary/benefits or even keeping him on the payroll if he is a thief, and just because he has HIV doesn't mean that he is protected from being fired for cause or even "at will". But neither do you want to wind up in court on a discrimination/retaliation action. Even if you aren't found liable for any damages under an ADA violation claim the litigation exposure could be substantial. You may also need to talk with your insurance carrier regarding this.

