A lot of fathers take a straightforward, honest approach with their sons when discussing the subject of sexual orientation toward homosexuality. They make it clear that they love and accept their sons, regardless of sexuality.
It is important for a son to know that his father isn't ashamed of him, and that he plans to stand by him. There are many stories of rejection, or even disowning, so boys are often afraid to open the subject. A father should take the lead, and let his son know that whether he likes boys, girls or both, he is accepted.
it is important for the father to talk about safety. Some people have the idea that because a gay couple cannot become pregnant, that gay men don't need to worry about protection. Condoms are still important, because a partner might have an STI and not know it. The son should be warned that it is important to protect his health.
It is also a good idea to discuss consent, and what constitutes a healthy or unhealthy relationship. Whatever a boy's preferences turn out to be, it is important that he knows how to recognize abusive or manipulative behaviors. It is important that he knows that he always has the right to stand up for himself and what is important to him, but that he should also take the time to listen to his partner and what is important to him. A healthy relationship of any type is important, and the roots of that understanding can begin with the father, and with an open and honest father-son relationship.