Also, is it the value that's the problem - if it was say £50, would you all think it was for the DP to buy himself a treat or should he put it in the commual pot?
Assuming the OP and her DP aren't in a bad financial position to have to spend all birthday/Christmas money on household bills/running, money gifts would come under the same heading as say, chocolate gifts. It would be nice if when DH opens the chocolates he was given at Christmas he would share them around, but I have no right to go open and eat them myself, they are his. If while I'm out he eats the lot, I have no right to complain.
It would be really kind and generous for him to say "actually, shall we get a new sofa/TV/pay for a holiday with my Christmas money for us all?" but if he wanted to say "I want to buy a new bike/some new suits/pay for gym membership for the year with it" then he's perfectly entitled to do that.
But as said, right now, the OP knows he got a cheque, she hasn't asked him how much for just snooped. She can't start a conversation at all about it until she asks him how much it was for.
If he lies, that's a different problem, but right now, he's not hidden the fact he's had a cheque, hasn't hidden the cheque itself and just not discussed it at all - I would say he's not really done anything wrong, although you might think the OP's FIL has done something wrong by giving a massively different value gift to his DS compared to his DIL. (I also have no problem with that, PIL spend far more on DH than they do on me)