Partner and Moving

My partner and I are buying a house together (we currently rent). I have 3 children from a previous relationship that live with us 4 days a week and he has 2 boys from a previous relationship who live with us 2 days a week. He has a lot more capital than me so will only need a small mortgage on his side, I will be putting less capital in and contributing more to my side of the mortgage so the house will be a 50/50 split.

Anyway, he has recently told me that he only actually needs a 2/3 bed house for him and his boys and could be mortgage free. He resents the fact that he will be paying half for a bigger (5 bedroom) house for all of us (meaning he would need to get a small mortgage) and that my children would all have their own bedrooms when his boys will have to share for the 2 nights they are with us. He feels that he won’t benefit from the house as much as I will.

I know he could change these thoughts but he doesn’t self coach and I just don’t know how to approach this with him.