1.Get him an straight A classe DVD, and let him to tell you how he feels about it. I recommand you Private's "Gladiators" (you can easily to get a copy from anywhere), haha, many handsome guys and beautiful sweeties!! And he has to find out which part is his piece of cake.
2. Ask him if he like to read some magsins like"Men's Uno" or "Men's Clothes" and you're gonna get him for some months, then to see if he was intrested to buy them by himself...
3. Check his personal computer, is there any nude girls' pictures? If not! Yes, he is absolutely gay no matter if he got some male' ones or not.
4. He spend most of his time to go out with his female classemates or friends, then he is gay oriented.
Anyway, my sister is the first one I talked of my sexuality, so show him your comprehension, give him some time and find a chance to communicate your own private life with him, he is gonna let you know everything:)