Why don't some people understand that cheating and sleeping with somebody other than your partner are different things.
Cheating is cheating, no matter when, where and is defined as ' a lie said or done'. You can cheat by sleeping with somebody (and then lie about it or simply hide it) or you can cheat buy lying to your partner regarding the recent big charge on the credit card or anything untruthful for that matter.
It doesn't sound like this guy is getting cheated on since he is aware and apparently his bf is not even putting an effort to hide or deny it.
Now my advise....
You have following options
1. tell him that you are not okay with an open relationship
and
a. he may actually agree stop sleeping around
b. he may say that he can't and if you are not okay with it you will have to brake up
c. he will say that he will stop (because he loves you) but will continue doing what he is doing now, except it will be cheating because he will have to lie
2. you decide that you love him the way he is and set some rules (as far as it goes to playing safe, getting tested, etc....
now option 1a. and 1b are very dangerous and if he agrees to 1a you there is a big chance it's going to be end up as 1c and it will always keep you wandering because people hardly change,
now you are left either with 1b or 2 options too....
Either brake up or accept him the way he is. If you don't want to hookup with other then DONT, but understand that for him sleeping with other doesn't mean anything emotionally and he loves you. The fact that he comes back to should matter more to you...

Accept his hookups as if he jacked off to porn... UNDERSTAND THAT sex for him is not a way of displaying his love and/or affection, at least I am talking about the type of sex that he is having on the side..
good luck