Your husband can see his father with his grandchildren outside of the family home.
I would not see him again under any circumstances.
I would not want him in my home and would absent myself and my children if my husband insisted he visit.
That type of disgusting behaviour is not acceptable and I simply could not be around it.
Husbands family, his issue completely to sort out....but not near you or your children.
This man is nothing to you or your children and that would be my position going forward.
No engagement or accommodations.
I wouldn't make a big fuss, it simply wouldn't be happening.
Several friends have dealt with difficult in laws over the years and they were calm, firm and utterly unweilding.
Contact simply wouldn't be happening.
It took a couple of years of this for the penny to drop, and in all cases, calm, respectful, very very distant contact was reestablished.
They were blood relatives though.
This man is absolutely nothing to you.
Cut him out.
Your son sounds fantastic.