This story breaks my heart. I think it's time for Mamabear to make an appearance. Lay out a boundary, calmly but forcefully. Do this in person, so he can see your eyes and your resolve:
"I heard how you ignored S3 last nite. He was heartbroken. If you cannot take even 10 minutes to spend with them when you come home, then you are no longer welcome to come home. I have tolerated your disrespect and coldness towards ME, but make no mistake, I will NOT tolerate it towards my SON. I suggest you apologize to him."
And then walk away.
Puppy
Puppy is very wise. He is giving you great advise here.
Different wording:
"I heard you ignored S3 last night. He was heartbroken. If you cannot spend even 10 minutes with him when you come home, then you are no longer welcome to come home. I will not tolerate disrespect in this house and I will NOT tolerate it towards my SON. I suggest you apologize to him."
"What is best for my kids is best for me" Amor Fati Link to quotes: https://www.divorcebusting.com/forums/ubbthreads.php?ubb=showflat&Number=2879712
Lay out a boundary, calmly but forcefully. Do this in person, so he can see your eyes and your resolve: 
