My dear young sister, nothing creates more marriages that never should have happened than premarital sex.
The people get "addicted" to each other, and they marry, only to learn that the incompatibilities, the character issues, the "red flags", all get
brighter, louder, and harder after the marriage.
He is a amazing person and I don't want to hurt him more, so should I leave so as not to make it worse?
Yes. It's clear to me from what you say that you recognize that he's not "the one".
Unfortunately, relationships sometimes end badly. They end worse after sex. But, I want to assure you of something. This "bad end" is like a toothache.
The pain will be more intense, briefly, as the tooth is yanked out. But then, healing can begin, because the rottenness is gone.
Don't settle for someone you don't want just because you don't want to hurt them. You are correct, in that you will hurt him far worse by marrying him and being disappointed in him than you will by simply ending the relationship.
A man is "made" to want deeply to be his wife's "hero", he wants to be her "best" choice. You will not, in the long term, be able to hide the fact that he isn't yours.