We live together and I think he's cheating HELP

Hey guys. I've lived with my boyfriend for a few months now and we have our lease locked in until the end of July. However, he's been really sneaky and I seen some messages on his phone that lead me to believe hes cheating. When I try to bring it up he won't talk about it.

What do I do?! Should I just ignore it? We're stuck living together until August 1st...

Help guys... what do you think?