Yes, I know what you mean. My son and my 2nd husband are just not "on the same page" about anything , so we rarely spend time with my son at all. I used to invite DS over for a meal sometimes, but the atmosphere, although polite, was never relaxed so those dinners have been cancelled.
I guess I'm lucky that , because I take my son's children to and from school every day, I do see my son on his own at his house and we have a chat.
Could you ask your son if he'd like to go out for a coffee or lunch in a cafe , just the two of you ? Or would your DH expect to come along too ? I know it's difficult when your husband isn't your children's father. You feel you have to be loyal to your DH but the deep emotional ties are with your children. And having your mother there too - you really are stuck !
I hope things get better OP. Sorry I'm no help .