No, he's not "experimenting!" That's the most ridiculous thing I've ever heard. If a 19 year old male was caught having sex with a guy then he's gay, plain and simple. Normal heterosexual males are not physically attracted to the same sex; that's not how our hormonally driven brains work. At 19, most males aren't even open to the idea of same sex relationships and, by nature, are somewhat homophobic. Now some here argue that he may be "bi". The odds of that being true are even greater.
Look, I don't mean to wander on the subject but the fact of the matter is that his actions speak for itself. As a heterosexual male, I can honestly tell you that I've never even given the least bit of thought of being with another guy. So, for him to have actually had an urge, or contemplate the idea, and then act on it tells me that your son is gay, that's a given. But the real question is: How are you going to deal with it and do you accept it? A lot of parents feel uncomfortable talking about sex with their kids for a lot of different reasons, which I feel may be the case here. However, what should be known, and the point I was trying to make earlier, is that people can't chose to be gay; it's who they are. The scientific explanation for this (neurobiology), which I support, is that you're born gay. It doesn't make you any less of a person.