First, as others have said, kudos to you for the fact that you have such an open relationship with your son.
Second, I'd be curious to know why you're so concerned about getting him to believe that oral sex is sex? It doesn't sound like he's using this distinction as a way to break a "no sex" rule. You sound like you're resigned to the fact that one day he WILL "go all the way." So is it really important that he agree with your definition of sex, or is it more important to you that he just know the dangers associated with oral sex? If it's the latter, then I'd concentrate on discussing the dangers rather than arguing over the definition. You could point out how certain STDs can be contracted through oral sex. Let's say his girlfriend caught something from giving another guy oral sex, had a sore in or around her mouth, the gave it to him (for example).
I'm 33, and as a teenager, I didn't think of oral sex as being sex. I haven't thought much of it since, but I still don't really think of it as sex. I know a lot has been made of the Bill (Clinton) & Monica scandal and him arguing semantics. In a case like that, I say, no way - you know what they're getting at. Oral sex is still "sexual contact" or "sexual relations" but perhaps isn't considered "intercourse" by many people. I consider the first time I lost my virginity to be the first time there was vaginal penetration, not the first time I had oral sex. So for those people who are adamant that "oral sex is sex", I'm just curious, would you then say someone is no longer a virgin after they've had oral sex?
Good luck to you. I'd focus more on the dangers than the definition.