My dear lady, if this guy went all "step back, sister" merely from you suggesting the "R" word, then there's nothing to blow. Or, in other words, you blew the lid off the truth - that's a good thing; you two are seeking different things. There's no shame in wanting a relationship, whether you seek one at 18, 28 or 38. There is shame, however, in trying to step back from how you truly feel and downplaying your interest in someone for their sake. It's not a place you want to operate from.
You're better off accepting this dude where he is at, and accepting the choice that leaves you with. Forget about his age, it's just a matter of you wanting more than a booty call, sex pal. If you do convince yourself that the price of sex with him is worth more than your relationship goals it will eventually takes its toll. You're asking for a long trip down the insecure and depressed highway. You're better off exiting and finding someone your speed.
He doesnt want you, Yeah its hard to acept that but you have to in order to move on. Why be with someone who clearly seems to dismiss your existence and/OR what you want with him? Hes not ready. Im assuming hes a really young guy, so let him go.