Child Stole from Grandparent

My question involves juvenile law in the State of: Massachusetts

Two days ago, I found out my 14 yo son stole $350.00 from his grandmother (my mother) whom we live with. He took it to his mothers for her scheduled visitation, and she brought him to purchase a bike with.
Since then, she was supposed to bring the bike back to my home and failed to do so. I told her it needs to come here and she said "You can get it when you pick up our daughter Sunday"
another part of this is, my ex wife tells me our son does not want to return to the custodial home (with me) and wants to remain with her. But I told her he needs to return.
Back to the bike issue... I am concerned something will happen to this bike and need it here as I want to return it to get the $ back.
Does this bike now apply as stolen property and can she be held accountable for not returning it to me instantly?
Thank you for any comments and help
There is a lot more to this entire story but, this question is only regarding the bike right now

(I'm not sure why your post is titled "child at risk")

The bike isn't technically stolen property. Mom isn't accountable here - your SON is the one who needs to be held accountable. Frankly, had my teen stolen from his grandparent he'd have a choice - return the property IMMEDIATELY or he'd be facing a nice long chat with the police.

He might be facing that chat anyway.

The title, I put due to the fact he has been in counseling for about 7 years now, as well as went through an outpatient troubled teen program last winter, he is being held back this year at school, and basically just a very long list of problems which stem from the influence of his mother.
My second issue with the theft is the fact she didnt even think to call me to ask where our son got $300 for her to take him to buy a bike for her house. He is definitely being held accountable but sadly he is with mom till Monday morning for her visitation. I have already spoken with an officer in my town and plan to bring my son to the station upon his return.
Thank you for the information. I was not sure if she could be held accountable knoing I want that bike here so it does not get ruined, as she is allowing him to ride it :-/

Ugh..what a mess for you.

At least he's in counseling though. And at least you're the CP...although you might expect Mom to argue that she can't be that much of a bad influence on him if you're the one with primary custody.

Will the store even take back the bike after it's been used?

Not sure, they said they would if the reciept was accompanied by a police report :-/
I just pray it does not get damaged in the meantime.

As far as me with sole PC... she gets visitations every other week from Wed through Mon morning, then opposite wks Wed, over night to Thu morning then Fri after 3:30 pm till about 8 or 9 pm so.. yeah horrible schedule for children, espcially during school year.
She has this poor boy so messed up its not even funny
She is suspected as being borderline personality disorder

thank you btw for the response, I just hope to be able to help my son through all this mess before its gone too far he can't get out of it, and also to be able to get the $ back on this bike. $ is a problem these days and that $ had a purpose to be used for