First piece of advice I would give you, about you giving advice to him, is to really think twice about all of your motivations for wanting to offer your son gay advice.

My dad was prone to advice giving, and when I was a teen I remember many times when he would give me advice about things like avoiding older men because until I'm 18 it is considered child molestation/rape....given the tenor of your post, I highly doubt that you would ever go there; however, the underlying point I suppose is to avoid advice-giving about specifically gay issues if your motivations are based in fear of some outcome that has gripped your mind concerning your son and his sexuality.

All that said to me as a gay teen was that my dad obviously didn't respect me nor believed in my ability to make good or even half-way decent let alone legal dating choices. It was as insulting as it was embarassing and it put up a wall between which stood until his death.

If you want to give your son advice, I would wait until he has first asked you for your advice. Then, let your answers come from love, and not from fear.