Do the best thing you can do: Talk to your son.
If there is gay-related porn on his computer, he is definitely interested in that aspect of sexuality. Since you have the websites as proof of his interest, you can acknowledge this with him.
He probably values his own safety and health by going online to satisfy his needs instead of running around with every guy he can find. Plus, the internet is a safe-haven for him because noone can hurt him or mock him.
I would approach him by asking him if he has anything he'd like to share with you, and to let him know that no matter what he tells you, you wont be disappointed in him or hate him. At that point, he may actually come out and just tell you the whole truth without you having to ask at all.
Hopefully you would be tolerant of his sexual identity, as he cannot control how he feels. (Trust me, I'm bisexual myself--- not by choice.)
What he will need is your support more than anything. The last thing he'll want is for you to be disappointed in him or even angry. He has a lot on his mind, such as worries about his future, about having children, pleasing his parents, and maybe even accepting himself which may or may not have happened yet.
Just be there for him no matter what. It's what he'll need.