Gah, biting is such a tricky one! As you know, DS is much younger than yours but he has in the past been a biter too If it were me (with DS older) I would have done exactly the same as you, after that if things weren't improving I would turn the ball around and tend to the other child instead. Fussing over the bitten child, checking if they are ok, making a generalised fuss. Basically ignoring the less than desirable behaviour but showing DS that when he bites he isn't in trouble but that he wont get the reaction he expects either. I know that is isn't very eloquent and I apologise but I hope I have made a bit a sense!
FWIW DS used to bite for attention, out of irritation, to get your attention but also he was 'over loving'. He would get so cuddled up and excited with love and cuddles he would end up nipping out of love he had such an overrun of emotions and didn't know what to do with them ITMS
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.
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