My kids do not understand boundaries and most social norms. They have an impulse to grab something and it is extremely difficult to stop them. We focus on what “to do” rather “not doing” - ask for the thing they want, etc. Its tough.

I don't think my twelve year old grandson would understand what stealing is. Sometimes when you see something he wants he'll take it, unless I tell him no. He'll remember that item a year later and if we go back to that store he'll go straight to it and pick it up if it's there.

Maybe he doesn't understand boundaries, social-perspective and discriminates between what's his and what's not. Or it could be an act of impulse. Either way you can reach out to your ABA provider for help as well as a therapist.