As of today, people has widely and openly accepted gay people in the world. Most countries have even approved same-sex marriage to back up this so-called acceptance. If you have a friend who is gay, you must make that person feel just how much you accept and support every decision he has made. After all, he is a human being too, who would very much love to have every right that other people, especially the straight ones do.
He has the right to love who he wants to love and work whatever he is passionate about. There is nothing wrong with him and it would really be great if you can remind him of that. No matter what happens, be there for him especially when he is doubting himself the most because of his gender problems.
Try out some happy birthday gay meme to greet him on his special day, to let him know that you care a lot about him.