my fiancé has changed

He threatens to take my son from me when he's born, he has also tried to say the baby's not his. I'm at my wits end please help. Is he cheating , is that why he's saying this ?(wev been together for almost 2 yrs) I cheated way back in the first week we were together(I was a drug addict and a alcoholic) I made bad choices. But I had never done anything since then, but since then he's talked to his ex several times , Iv caught him with a secret Snapchat and he had a coworker on it saying she was pretty, also he messaged a girl I heard him call baby on Xbox chat. I just give up. Please someone help me.