He let you down once, he will do it again.

If you let him come back all he will learn is that he can walk over you whenever he wants, you will always be there as a second option.

Just think... if you were so great together, why did he cheat on you in the first place?
Such a thing is NOT a "teenage mistake". It is a conscious choice he made, knowing full well that it would cause unimaginable pain to you.

In my opinion the guy is a flawed character. I would never trust him.