My DP used to complain about being spread too thinly and I had to point out to him that this was because he was trying to be too many things to too many people: marriage counsellor to his dad and surrogate husband to his mum.
I reminded him that his primary roles are dad to his sons, partner to me and step dad to my DCs.
Yes his parents will always be his parents, but the generations are as they are, and he is not responsible for his parents' marriage and happiness.
Of course when needed, we will both be there, but day to day we have to focus on OUR immediate family (us and our respective kids).
As a blended family, it's even more complicated.
But the fabulous book Stepmonster states that the primary Union is husband and wife (or partners as we're not married!) and everything else has to stem from that.
For you, you need to be his number 1 priority. It's the way it should be.
His mum should expect that too. If she doesn't, he needs to steer her there.
I think you'll have to point this out to him. Hope you can make him see.