My little grandson, then 4, sobbed every time it was time to go home - because he cried so much for his daddy his mother stopped all contact for him and his sister and the boy was punished for wanting to see his daddy. When my son went for his usual Wed to pick the children up the maternal grandmother was holding my grandson back at the top of the stairs while he was screaming for his father. She shouted 'You will never see your children again' Extreme I know but he hasn't although he fought through the courts, 4 times the judge told the mother that my son should have contact she never allowed it. Sorry to go off on a tangent but it brought it all up for me. We haven't seen the little ones for 5 years plus now. My son was in the local town just before Christmas when he saw his ex-wife coming towards him with the little boy -she shouted at the child to FACE THE WALL - which he did, this so he wouldn't look at his dad. There is no answer to this at all for us and we really miss them.
Jadey, such a sad story - must be truely horrid for you to watch those you love suffering.
I dont think you can have much influence with how the couple sort themselves out - thats up to them but you can support your Son in being the best father he can be and be a warm loving Grandma where your little GS can perhaps let of steam a bit as he gets older. Children need to have a listening ear when such events happen, and he will appreciate a place were he can have a good cry or moan without feeling he is upsetting his parents.
I suspect your little grandson is now old enough to begin to understand a little more about his life and thats why he is now getting upset. It will take a little time, but your son will need to see his boy as often as he can to spend time with him and let him know he loves him. As he gets older I am sure your little GS will begin to understand that although his Mummy and Daddy dont live together they do think the world of him. For what its worth I, personally, would be against them getting together if they really are not a happy couple, but thats just me. As with complex family arrangement - we've all had them - it can all seem pretty bleak but they are usually just stages that have to be endured - and they will pass. I wish you all the best.