I don't fully understand the reason why people become gay, other then knowing it is psychological rather then biological. Explain to him that, there is really nothing wrong about been gay. Explain to him that a person can only be at wrong, when he conduct a behavior that effect another person's freedom and that is still circumstantial base (e.g. if it is a cop arresting a serial killer, then the cop is not at fault, despite his behavior effected the serial killer's freedom) Been gay, doesn't really effect the freedom of others, so there is nothing wrong about it.
Some people felt been gay is wrong, because they believe it is not moral. I am not going to go into a debate about the moral of homosexuality, but personally, I believe whether something is moral or not, really is irrelevant to determine whether a behavior is wrong or not, as lack of moral, do not necessary effect freedom of others, explain that too him also, that is especially if he also feel been gay is not moral and want to be a moral person.