Engadged but feel like a single mom

I have an amazing fiance we been together for 2 years. But my son and him dont get along like I would like them to. At home he doesn't acknowledg him or ask him how his day was or nun like that barely talkes to him or doesn't even like to do activities with all 3 of us invloved unless its a video game (we don't play ever day). My son is 4 he likes him more then his own dad but doesnt talk to my fiance much either. I do want them to be like step father and son. But I think fiance feels uncomfortable or whatever cuz he has 2 other kids and doesn't want to feel like my son is taking there place but it affects us all I think in some way huh *sigh idk... Ladies what do ya think? I talked to him about it before thatbit bothers me but he doesn't really try to bond with him....