How do I tell this guy to go away?

So there’s this guy who likes me, I had no idea he did as he was a patient at physiotherapist I was intern at. We use to talk and he was a very friendly guy like with everyone. Even the head doctor got liking to him and use to insist that he has lunch there with everyone. He was flirty with me but I didn’t think much of it.

So I left for Canada and left office. Then one day my friend told me he was asking about me like if I’m doing fine with this while corona thing and if I’m ok. Then my friend told me he was asking for my number and said ask her if she’s okay with giving me her number, I gave it.

He texted me later that day and was being all friendly and flirty , I was just wondering why does he compliment me and say he missed me. So he got bit strait forward and I told him I have a boyfriend, he said he understood and I apologised and he said it’s cool and wished me luck , I said thanks.

Then he asked if we can still be friends if I’m okay with it and I said sure. He texted me 2 days later saying if I have any movie recommendations and I said no I don’t watch movies and he said “cool I’ll just watch looney tunes.”

Then again today he texted me “Whatcha doing?” I said “nothing” He said “sounds thrilling” I said “yep” He said “ I thought I was having too much fun, I’m jealous” I said passive aggressively “ I’m haaaaving soooo muuuuchhh funnnnnn, it’s not like this when I’m talking to someone but it’s a pain in the butt rn”. He said “same here, it’s a hail storm going on here” I didn’t reply.

What am I supposed to do with him??????