My partner and my son's relationship

My and my partner are together 4 years we have a beautiful 1 year old boy and he's great with him. I have another son from a previous relationship who is 7 now. My partner and my older son somewhat get on together but they have never been very close. I guess my partner didn't want to step on the toes of my son's dad. I thought their relationship would blossom over time but it really hasn't. Don't get me wrong they do get on quite well but they aren't close for being together 4 years and having another child together I thought our family would be more 'together' than it is. I'm desperate to get them bonding but I don't know how ?!!