I'm not sure there's much else you can do. Being right there and stopping him is the best strategy, probably, but unfortunately it's his siblings who will cop it mostly. It's a horrible feeling being *that* parent, so you're doing a great job to keep on top of it. I think they tend to just grow out of it with a bit of time.
It's good to model alternate behaviour, and to talk to him about the way to behave, but with little kids and their low (no!) impulse control, it really does come down to physically stopping them much of the time until he gets a bit older and can begin to regulate himself.