He’s 15, is this really the hill you want to die on with him? To be honest you sound like a petulant child who’s not getting their own way. He ate some chocolate, who cares? Take the rest and hide them and eat them yourself if it means that much to you. Oh and take the other box as well then you can have lots of chocolate just for you. So yeah YABU.
It's not really about the chocolate...
I wouldn't do anything petty like replace his trainers, but I would quietly and firmly keep insisting that the chocolates are replaced.
To clarify, I wasn't suggesting that OP should take his branded trainers and replace them with cheapo ones; I was responding to a PP who was suggesting that a 16yo somehow wouldn't understand the difference between an expensive version of X and a cheap version of X. I was thinking more if he had Nikes that got trashed by a friend's carelessness/deliberate actions and then the friend, when forced, 'replaced' them with budget ones!
But as I always say, the gift itself is a red herring. The issue is that he saw someone else's (unopened!) gift and decided to take whatever he fancied. He did something really rude, didn't apologise, then doubled down by buying a cheap replacement that basically says "I'd rather not spend my money on you".
It doesn't matter what he took, it's the principle of it.
Nailed it. The posters saying "but it's only chocolates" are spectacularly missing the point here.
Am I the only one wondering whether OP's DP bought the presents from the teenagers - as you would expect if they were toddlers - as he knew that they value their mum so lowly that, if he hadn't made the effort, she would have ended up with the first thing they could find in Poundland, rather than them 'wasting their money' on her, by getting her something special that she would really appreciate?
Yes, it's the thought that counts - but not when somebody deliberately put no thought into it whatsoever.