"I want to believe him, he seems like such a sweet person, he has nice friends, his girlfriend seems great, but..."
Just because he seems like a sweet person doesn't mean that you need to date him.
Just because his friends are nice doesn't mean you have to date him.
Just because some other woman likes him enough to be his girlfriend does not mean that he is right for you.
You don't need to dig up a REASON to not date him. If you aren't feeling it, that is plenty reason enough!
You aren't being mean! You are leaving him enough free time that he can find and fit someone into his life that really wants to be there!