I'd suggest telling him every single feeling you are dealing with and don't hold back one little bit. Until a BS can believe the WS truly understands the F'ing hell the BS went thru / is going thru, R cannot happen IMO.
You have NO duty to spare/protect his feelings. May sound cruel but I felt better when my wife would curl up next to me in bed and ball her eyes out, tell me how horrible she feels about herself, how horrible she feels about the pain she caused me. At times I had ZERO desire to do anything that might make her feel better. I could have, I knew what I could do/say, but honestly IMO she needed to feel like s*it about herself.
For a while I just laid there, letting her cry, not saying a word, because in the past (pre affair) my instinct had always been to do something to make her feel better, to help, to fix, sometimes at my own expense. Now, I consider myself first, my needs/wants/desires. At times this makes me feel selfish but when you constantly set aside yourself for another you are eventually taken for granted :/
"TBH, at this point in our relationship, I just do not have the energy nor the desire to help you deal with your pain of your mom's passing. You will NEVER understand how abandoned I felt. You will never understand the HELL I went thru, dealing with our newborn AND my father's death while YOU were off banging your girlfriend.
For now and the foreseeable future I will be focusing on taking care of myself and our child. I will be making changes in myself and this 'relationship' that are in my best interests. How you deal with those changes, that's up to you, but your needs, wants, desires, etc are VERY far down on my list of priorities and will be for a VERY long time.
So, sorry about your mum, but I have to put myself first for now."
He made big boy choices, now he gets big boy consequences. I would make it a priority to look him in the eyes and say "I never got to mourn my father's passing because of you. Where were you the day my father died? (Make him say where he was) And what were you doing? (Make him say it)
So you asking me to help you with your mum's passing, to comfort you, I'm just not capable of that right now."
You have no duty to set your feelings aside for him. I would suggest going to the funeral service together as a family though. His mum wasn't part of the affair and assumeably she deserves the respect of your appearance