In like with a guy who has a gf?

i'm 22 and i have a lab partner in college who's 33. he has a girlfriend who is his age but shes older, not nearly as good looking, and as far as he tells me doesn't really connect with him on an "emotional" level...however he is not really the emotional type (spent a lot of time in jail) :/ it's a pretty complicated situation...but basically we flirted for 4 months and then when the semester ended he made a move and kissed me. he seemed a lot more interested at first but a month later we have had sex once and i'm nervous he's disinterested and i gave up too much. whenever i bring up his gf he brushes it off but i found out he changed my name in his phone bc i texted him once and she saw it. a lot of shady stuff goes on but i really like him and i just want to be a little more than this....also i feel like i can't bring up the fact that these things bug me because i feel like i'd be putting myself out there too much. basically, how do i get this guy to break up with what he says is a dead end girlfriend of four months? and what should i do if he wont?