I suspect your son already assumes this gentleman is your boyfriend. I don’t think you need to make any big announcement about this fact. Your son sees that this person is important to you because you’re spending time with him, and he’s the first person you’ve dated that he has met. That’s probably enough for now.
Follow your son’s lead about how much information to provide. If your son asks, “Is he your boyfriend?” of course you should answer honestly, “Yes.” His next question is likely to be, “Are you going to marry him?” Again, try to answer simply and honestly, even if your answer is “I’m not sure.” If your son doesn’t ask, then he probably doesn’t want to know any more information than what he sees. There’s no need to say anything more unless there’s a change in your relationship that will directly affect your son’s life.