How to tell him that I have someone else?

I had a relationship with a guy who is 4 years younger than me. But he is always busy and almost had no time for me, even just to text me. I was thinking that because he is med student. So was trying to understand. Sometimes he didn’t text me over than a week. But someday he disappeared, he didn’t reply or answer my call. So I thought he just didn’t take me serious, because i am already old. So I forget him and move on.

It was already a year after I met him the last time, and I already have boyfriend. And yesterday, after work he came to my place, he waited for me in the lobby in my office and tell me he was super busy and but he told me he is a doctor now and sounds so proud to tell me that. And like nothing happened.

How to tell him that i am already have someone else without hurting anyone? I’m afraid it will disappoint my boyfriend if he finds out about my relationship with this guy, because he doesn’t know about my ex.
How to tell him that I have someone else?
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