Barbers hang out with other barbers,
Doctors hang out with other doctors,
Teachers hang out with other teachers,
Sluts hang out with other sluts...
Cheats hang out with other cheats. He has friends that have no morals, and sadly he doesn't have any either, otherwise he wouldn't be hanging around with them. I personally think he has already cheated on you...but you have to find that out yourself.
Explain to me, if he's in a relationship, why would he go out clubbing and grinding on other women when he has a girlfriend?