Don't know if I should leave him because I'm not sure if he cheated!

Ive been with a guy for a few years now. The first year of our relationship wasn't serious- we always went out in groups and were never alone. He never showed emotion towards me, and didn't seem to really care about my feelings.. However, the past year we only go to dinner and movies by ourselves and he has completely changed. He only wants to spend time with me.
Recently, I just found out he's been speaking to 2 girls behind my back for most of our relationship.. Girls that I had NO CLUE about.. when I first confronted him, he completely lied and said he hasn't spoken to them for that long. He said they were random phone calls just to say hi because they were old friends. He says he has never seen them in the 2 years we have been dating either.. He cries that he can't live without me and that I shouldn't leave him..
I don't know what to think about this. HELP!