I don't think it's necessarily child abuse to give a teenager a sip of beer or wine or whatever. I think your son would find it traumatic that his ALCOHOLIC dad would encourage him to drink--it's sort of like saying, "C'mon, kid, be a loser like me." Ugh.
The bigger issue, to me, is that it upset your son, and your son is finding the visits upsetting. This doesn't have to involve your trying to show he's not fit to visit his son. I think his son's wishes are enough, standing alone, to justify not making him go.