This might have started because of your BIL and maybe not. We don't have anyone who chews tobacco in our home. Gotta say I'm grateful for that since I remember family who had that gross habit when I was little. Yuck!
Anyway, if your son looks up to his uncle, it's possible that he could be doing it to mimic him. But not necessarily. My two year old has started spitting and I haven't seen anyone in the family doing that. Perhaps the older boys when they're outside?
I've scolded my son when I've caught him doing it inside (usually when he's mad about something) and threatened to take away something (toy, TV, dinner, whatever) or have a time out. I make him apologize and he gets the one freebie warning since he's two. If he does it again, I do whatever appropriate punishment I promised. That's enough to make him not do it inside. If he's outside or spitting in the sink like when he brushes his teeth, then I don't care.
If I heard him spitting from another room, I'd do the same thing. It'd make him think twice over spitting when I'm not around and it'd make him think I could see what he was doing when I wasn't around. The magic of mommies...scary! LOL