Hi Kate13, I remember you from your previous posts about this situation. I'm guessing that now your daughter in law has moved out the reality of the separation has finally hit your son and he's grieving for the future he recognises he will no longer have with his wife.
Sometimes all our children (in fact any of us) want is a listening, non-judgmental ear and to share the burdens of life. The problem with that comes when we, for whatever reason, end up carrying such burdens alone. In your shoes I would be glad that my son is giving safe expression to his feelings, but as your husband isn't supportive I would be looking for a friend to offload to. (Gransnet is a really good place to find support too.)
In practical terms your son (and the wider family) have all been hurt and I know it seems trite, but time really does blunt those feelings eventually; at least enough so that a 'new normal' life can be established.
I think I remember that your son sometimes travels abroad for his job, so any kind of shared care for your grandson may work in his favour at times. Also, when he's at school full time there will be opportunities for your grandson to maybe spend some of the long summer holidays with his dad - especially if your daughter in law is working. There are some wonderful holidays organised for just such times here: www.careforthefamily.org.uk/Events/Family+Breaks/Take+a+Break
Keep coming back and sharing until things are less raw at least. (((hugs)))