Its fine to have fun with this guy but don't expect him to view dating as something sexually exclusive.
He has already shown that he is not interested in monogamy and having a girlfriend does not deterring him from hooking up with other girls.
I don't think what he is doing is cheating because dating does not have the same official status and automatic expectation of monogamy/sexual exclusivity as marriage or a civil partnership does.
But old habits die hard.
If he likes to hook up with several extra girls when he has a girlfriend, he will likely do that if you are his girlfriend.
If he likes to lie to his girlfriend about hooking up with other girls, then he will lie to you the same way if you are his girlfriend.
Sounds to me like you would be better off sticking to no strings or fwb with him and never expecting anything else.
That way you are both clear that there is no point lying or pretending to be in an exclusive relationship.