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."
Lay out a boundary, calmly but forcefully. Do this in person, so he can see your eyes and your resolve: 
