Porn, however legal, is often based on exploiting someone's vulnerabilities, so we would advise him against that. Not sure what we would think of attractive non-porn like clothes modelling shots or naturism. One never knows here whether porn means porn, or any and all erotica.

Anyways, having advised against the porn, then what? What does he do with his seminal fluid buildup? Do we advise him to wait for wet-dreams, as the heads of Catholic monasteries did? That's not ungodly, but as a self-affliction, it's as least as hard going as self-flagellation with a cat (of nine tails, no relation to the one on this site). And it's an asceticism that has no definite biblical support, as many people have pointed out.

Of course there are much more worthy things he could devote himself to than taking care of his prostatic needs, but something has to happen at some point.

What do you advise your gay friend about that? Get married to a woman who knows he's gay but is willing to be his source of relief?

Actually, it may not be necessary to advise him on that. If he discovered how to express his God-given Christian love for others, even if no sex was involved, he would have something much more exciting on his mind and would forget to tell you anything about how he looked after his sexual distractions.