Shocked your mum didn't even try and wake you and let him do that. She must know how horrible that would be to miss surely!!! Has seh apologised!? And have you made it clear to ds that you know he was excited but that you are a bit sad that you missed the excitement and presents are only to be opened when everyone, especially mummy is there too and please can he remember for next year as Christmas is about families being together and that includes at present time too. And make sure your mum is in earshot when you say that to him so she is in no doubt how out of order that was!
And of course he can wait if that's what your family tradition is (and you should start that asap I think). Or at the very least he can come wake you and hurry you along if you aren't awake!
In this house ds opens his stocking when he wakes, but... he carries it into my room and opens it in bed with me (yes even if that is at 6am or once time at 3am!) Then we go back to sleep for a while and/or he can play with whatever toys and games were in his stocking.
Then when we get up (which is usually earlier than I would like) it is one presents each from under the tree but only when we are both there. Rest of presents are only allowed once everyone is showered and/or dressed and we have had breakfast together. Table is specially laid out with crackers and his xmas bowl and mug and he gets a variety cereal pack to pick what he likes from (which is special as he doesn't get them often otherwise).
So it's exciting and special and is something to look forward too in itself, not simply a present fest first thing on waking!
It's always been that way here, so he sticks to it. Sounds like you need to start something similar pronto.
It's done now though, so you should at least try and enjoy the presents with him and go spend time with him now.