The Way I Hear It : Heads or Tails?

Surely you don’t need to spend time with your ex or listen to his concerns if you don’t want to? Keep your distance.
Is there a reason why your DiL is so reluctant about letting her child spend time with his Grandad?
I understand you are hearing the same thing from your son but I think you have to tell him you can’t get involved- neither between him and his wife nor you and your ex. Just enjoy your grandson.

I don't understand with threads of this nature why one parent is able to insist who can and cannot see the child.

If your son wants his father to see his GS then he should ensure he does.

I think you should ask your daughter in law why she’s worried about her child safety.

Your son has to deal with this problem. His wife is refusing his father access to their child, and your intervention would serve no practical purpose.
The reasons behind the daughter in law's action are important and again, only your son is in a position to deal with them. Possibly, having been the child of a broken marriage he does not want the same to happen to his children, and this is where his wife has control. Saw it happen with my best friend and her unreasonable daughter in law; took seven years to resolve it.

No one knows any details of the safety issue your DIL has so I’m not sure why anyone is making an assumption about what the son needs to insist. Behaviour we are used to from our families of origin may seem perfectly normal to us, but outsiders do not have the emotional attachments to dismiss certain redflags. We don’t know if this is the case with the DIL. This is as possible as the automatic assumption that the DIL is simply being unreasonable here. Either way, your son knows the details. Up to him to navigate this situation.

Your ex is bang out of order insisting on your involvement here. Not your issue at all. He needs to speak with his son and DIL. The fact that he insists you take his side lets me know at the very least maturity is lacking.

Your son, I understand his need to vent but it is okay that you erect boundaries and let him know that his offloading to you instead of dealing with the issue himself is becoming a bit draining.

Protect your peace. This drama isn’t yours to get immersed in.

I would remind your son that you are DIVORCED and that it’s up to your son to work things out with his father and wife. You haven’t mentioned the crucial part of this carry on….WHY won’t your d.i.l let your ex see his grandchild??