I think you were a little unreasonable, in that your ex is as horrified and worried about the situations your son is getting into, as you are. 15 year olds - be they boys, or girls - don't think longterm, I'm afraid. They make stupid decisions/choices and often feel that the consequences are unfair... so won't/don't abide by them.
You say that your son was in school yesterday, and should have gone straight home to your ex. Who was obviously expecting your son to do precisely that, as he was there "cooking dinner". Which sounds as though your son was the one who took it upon himself to decide not to abide by the consequences (grounding) of the choice he made (to stand by and not even try to stop the assault - I really hope the shopkeeper is okay, and has no long lasting damage, physical or mental, from the assault, by the way).
Your son chose not to go straight home, presumably without informing his father of this decision, as he was "cooking dinner".
Your son made the decision to take things which didn't belong to him. He chose to steal. He made the choice to shoplift. Again, presumably his father knew nothing of this until it was too late. Was your son caught, by the way? How did you and your ex discover that he had made another horrific decision for himself that goes against social morality? Is/will he be facing a charge for his choice?
It might be that he's "acting out" because of your divorce. It might be that you're right and he's cottoned onto the fact that his father is a softer touch than you are, regarding boundaries, rules and responsibilities. It is, however, more likely that he's simply fallen in with "a bad crowd" - that his friends are, like him, incapable of thinking longterm and desperate for the brief thrill/adrenaline rush that behaving like this gives them. Behaving like this might make them feel powerful. Perhaps, like your son, they come from broken homes (and I'm not judging you, OP, because my son also comes from a broken home... and I have the teen years, for him, yet to come).
My 20 year old used to hang out with a crowd of girls when she was your son's age... until they were caught shoplifting, on a day when she wasn't with them. She stopped hanging out with them, immediately - which was her choice, because I explained the consequences of such behaviour in and around society, longterm. How would your son feel if he was unable to find work, because of a stupid choice he made as a teenager? It happens. In this climate, employers can afford to be picky about who they hire - and a prior conviction for theft? Well. I'd not employ him...
I think that, rather than screaming (and, again, I understand why you did that, completely, because I would have wanted to do the same) at your son, you need to talk to him. Find out why he is behaving like this, determine if it's because he wants to fit in with his friends, try to explain to him what he is facing in his future if he doesn't start to make better choices and... I think this is going to be the tough bit... start co-parenting properly with your ex. Because in this situation, your son needs you both to be his parents, I'm afraid, rather than individuals who are looking to score points against one another.
Good luck, OP. I have a feeling you're going to need it.