I get you about wishing you'd neverheard about Asperger's - the only trouble with that, is you would still be in the dark as to why he behaves this way, because the label didn't change him, it only changed your understanding. I hope it helped, not hindered.
The age gap is a problem, at this age. As they get older it will be less of an issue. I have 10 years between difficult child 1 & difficult child 2, I remember difficult child 1 used to be sad because he couldn't get on with difficult child 3 the way easy child did. difficult child 3 was only interested in difficult child 1 sometimes, when it was play time and they were tickling or wrestling. Other than tat, he didn't like to be around him. But easy child - she was favourite.
Of course now, difficult child 1 is Superman in difficult child 3's eyes.
A lot of problems came about because difficult child 1 was so much better at computer games than difficult child 3, purely because of age. So I had to ban certain games from difficult child 3, at least to not play them against difficult child 1
But games which allowed different skill levels for the two players were more aceptable. Then difficult child 3 rapidly caught up to difficult child 1 in skill level, and a lot of the problems stopped.
For someone with Asperger's, it's VERY important that everything be fair. They really HATE injustice in any form. Always losing, or losing by fluke when you really have worked hard at it and thought you were winning, will set off a tantrum in a kid who hasn't got the social maturity to handle it.
There ARE some good games they can play together. Games like Mario Party, which are heavily based on chance and have different skill levels, are good for a group to play together. Also, he and his brother could each play Or at least barrack for) two characters which increases the chance that at least one of them will be doing OK. It's also important for both boys to show respect to one another, which means asking before assuming, never just grabbing (although the impulse control stuff is always a huge issue). And they will get it wrong, and there will be rages. Sometimes the best thing you can do, instead of try to hold him, is to walk away, everybody, until he can calm himself down. THEN maybe talk about it. But observe him, see what has triggered him and what helps him calm down.
Watching a movie together was good - you changed the activity. His intial resistance would have been to the change, because in his head he wanted to turn back the clock to where he was in the game and winning. But life isn't like that. he just needed to wait for his head to catch up to circumstances.
I'm glad it eventually sorted out and quieted down.
Marg