Dating a man with a baby on the way

I have been dating a guy for 5 months. He told me when he met that he has a baby on the way from a woman who he saw a handful of times. She basically turned round and said she couldnt get pregnant, then it turned out she could and shes already 41 and.this could be her last chance so she decided to keep it and he can be involved if he wants to. The baby is now due this month and I myself have started to freak out about it wondering if i should run for the hills. Few points - he is a nice guy albeit immature given how the baby situation has unfolded and has accepted me as a single parent (son is 14) and wont abandon his own child. The last 5.months have made me happy but obviously things will now change as the dynamic between us will be different. Also baby mum has now started with the drama and puts him in a mood which he sometimes takes out on me. Anyone else been in the same situation with a positive result.