First of all, I have no professional experience in the field, only my own thoughts and feelings.
Make sure he knows you're there for him and that you're truly okay with it or he might see it (or himself) as a burden to you too. Try to get him to do activities he likes and as much as you can try to keep him away from negativity. Bad comments, really sad movies and such can really do a lot of harm.
If you can, subtly talk to him about successful and well seen openly gay people, try to make him see that whatever his dreams and hopes for the future are, they are not incompatible with being gay. I really don't know what else might help but I do hope everything will get better.