Young, inexperienced, socially awkward guys (or any combination thereof) tend to get stuck on cute, confident, decent women (decent as in "treats guy like a fellow human being"). Sometimes, I question whether or not I should just be a huge cunt, but I can't do that unless someone crosses a line. If they don't take no for an answer, throw the bitch switch and make sure you've gotten witnesses or another indisputable record of a demand that he doesn't contact you via any medium, and if you work with him, make sure someone else is present or that you have a recording device, should you cross paths.
I've had run-ins with creeps that took me MONTHS to get rid of. I'm talking 50 texts in two days, multiple blocked calls (sent straight to vmail) and texts every day for months, notes left on my car (stating that my comings and goings were watched), firecrackers set off on my porch, creeps at work, you name it (these were all different weirdos; in one case, it was three at a time). Some were younger, some my age, some older, but the common thread was social awkwardness not immediately evident, resulting in the creepers taking a "Hi," in return as "OMfG she acknowledged my existence, she must be interested! Must start stalking campaign now!"
As a result, I'm icy and curt, but not quite cunty... until the line is crossed. It sucks to be/feel compelled to change, because people are dense as fuck, creepy, entitled, or just inconsiderate and disrespectful of your autonomy.
I know you don't enjoy being an asshole, and you certainly don't want to hurt anyone's feelings or be the person that ruins someone's day; but if you need to get rid of him (the squicky gut feeling seems like you do), throw the Bitch Switch (obviously without making it personal), and let this guy know that you're not interested and you don't want to be friends. End of. If you "have a boyfriend" or "have something else to do that you can't get out of," he'll just think that those are the only things keeping you guys from happening as a couple; that you really like him, but you're taken, busy, etc., but things might eventually change.
The fact that he gives you the creeps, even if that feeling ends up being unfounded, is your gift of subconscious behavioral observation trying to keep you safe.
NEVER ignore that. Society has conditioned women to "be nice," to smile on command, to always accept the kindness of a stranger (even as they invade your personal space and attempt to build a false comradery), or to feel sorry for the loner (lots are loners by choice, or just introverted and picky about the company they keep - nothing wrong with that). Well, fuck society right in the ear.
After you tell him to buzz off, without justifying your decision, be ready to go hard, just in case he doesn't take no for an answer. It usually doesn't come to that. Mostly, no response is quite effective, as is changing one's contact information. Fake names for social media and email addresses that don't link to or contain your name, as well as not posting pics at all (or not without strict privacy settings, even then, it's iffy) work well to avoid the technically unsavvy cyber stalker.
Please forgive the hypervigilance, if it's a little much. I'm just that way bc I'm a creep magnet, lived alone for a long time, and have been scared shitless or highly pissed by weirdo shenanigans. Never, ever feel bad for trusting your gut, asserting your autonomy, or your right to a personal life and circle of friends of your choosing.
Be safe.