Personal Question Any help would be appreciated?

I am 29 years old, I have a two year old son. My husband ped away and i am now seeing/dating an ex of mine. He has asked me to marry him now. He is pretty insistent on trying to make me change my sons last name to his when we get married. My husband and i had alot of relationship problems and only stayed together cause of my son. I am now pregnant and he wants us all to be a family and he doesn't want mikey my son to know he isn't his real father and he tells everyone that mikey is his. My son is named after his father and is the 3rd what should i do. Should my son keep his name and be the only one that has a different name or change it. I want it to be in his well being. Please no rude comments