first off, I know this will sound bad but, don't have unprotected sex with him. if he has a rep for 'getting around' you don't know what he might have. I know it's not the advice you're looking for but it's not worth it to get an STI.
I would say, make your own mind up about him. My philosophy is, if you don't see people more than once a month, why should you give a s**t what they think about you or who you're with. They're not a big or important part of your life so stuff them. See how it goes :)