I feel in the way

Ive been ‘official’ with my boyfriend for 3 months, Ive met his immediate family and have them on my Facebook already. As far as new relationships go it’s been going good so far, except one thing. He has a child with his ex…they still get on really well and are still in contact outside of parenting. They joke with each other and even though it’s brilliant that they get on for the sake of their child I still feel some type of way. Also his ex is still close with his mother and I kinda feel out of place as if I shouldn’t be there idk. Btw this is NOT me being selfish over the relationships because I love the fact that their child is growing up in a healthy environment, I just feel like maybe I dont belong. But he reassures me that I have nothing to worry about and he’s with me now but still there’s something I can’t put my finger on. Did we move too quick? He split from his ex and got with me straight away and then quickly introduced me to his family so am I a rebound?