Getting Closer

[LittleMonsterMike]

OK so i my best friend and i started having sex last week was the first time and I'm going to his house again this Monday and he has expressed that he wants to have sex again but i don't just want sex from him i really want to try getting closer to him then just a fuck buddy. He currently has a girlfriend and i don't mind that but i don't know if he's bi and actually likes me or if I'm just an experiment. I do i get closer to him with out freaking him out?

[Perseus]

Tell him that you don't want to be his fuckbuddy. I doubt he's gonna flip his shit over you not wanting to have sex with him all the time, lol.

[Fact]

First of all, try not to get too involved if he has a girlfriend that he's serious with? Could make things awkward.
Secondly, the fact that he has a girlfriend and is having sex with you suggests that he could be using you, but it's not quite clear from the information you've given.
Only way really is to communicate with him about it, ask him how he feels about you, not about having sex with you and see if he actually cares or if he just wants to use you as an experiment.