Kenneth, you dont mention what your friend might think. You dont mention what he knows about his dad and this subject. Certainly it could come as a very complicating shock to a guy who discovers his dad is gay and getting together with one of his buddies. Do you know whether your friend believes his dad might be gay? You list yourself as 50% straight, 50% gay. How does that play with respect to this particular friendship? Is this particular friend gay? Does he know that you identify as 50% gay at least in one place?
If there is a lot of doubt as to how your friend might take it, I would not go near a relationship with his dad. It isnt just that it might risk your friendship, which would be bad, but I think it would be a very undermining thing to do to a guy. If one day his dad comes out and he finds out hes in a relationship with some man hes never met, that would be one thing for him to accept. Coming out of nowhere, it being one of his friends [I assume you guys are good buddies], could make him feel very betrayed.
You owe him better than that.
On the other hand, the age difference doesnt seem way out of line, even if hes 16 or 17 years older than you, at your respective ages it wouldnt be too remarkable. What is remarkable is that your connection is his child. If your friend knows his dad is gay, knows you are gay, isnt wanting to get something going with you himself, gets to see the two of you becoming closer, and if this is something you really want, maybe he could be relaxed with it. But you would have to do it carefully. And those are a lot of ifs.