Sports should be entertainment, not stress. I admit, I stress out during games too, but I don't yell at the TV or indicate that I'm angry. I make sure my stress is my own and doesn't spread to my wife and son. That's how my Dad watched sports, so I learned it from him. I figure that the team is no reflection on me - I can't take credit for the wins so I shouldn't feel shame about the losses. Yeah, I'm bummed when they lose, especially in the playoffs, but I can't let that ruin my life.
I guess my point is that the Bills are winning much more often than losing these days, so maybe just teach your son to savor the wins and shrug off the losses. He might find being a Bills fan much less stressful.