His father needs to spend some regular quality together on that basis , just feeling guilty on his dad's part, is not helpful beneficial really,
Look at doing interesting stuff together as a family sometimes or on a more regular basis
aswell as spending quality one to one with him too
Also can you encourage him to get involved in friendly way in family stuff , so he doesn't feel unwittingly pushed out,
So makes him feel bit more of family unit, not like a spare part of this family unit,
I mean ask for his view points in regard of family life for his input on stuff such as for e.g decorating the house, and doing cooking 🍳 together quality time with his dad or doing helping to do Diy/or and gardening with Dad and yourself as it's often easier to talk about stuff personal stuff whilst doing things,
It does not then feel like you being interviewed kind of feeling,
If you don't have a garden?
Put your name on a waiting list for Allotments garden?
Respect his viewpoints but also important to ensure you as a family have boundaries, based on respect and harmony at home,
But also realise Op, and respect that he needs quality time space for himself too,
It's finding a balance
Find out what stuff , what kinds of activities is he interested in or and curious in,?
So he can join clubs ect