Would you be concerned about DGS doing this?

If it were me I'd tell the child that I had an illness that causes a lot of headaches.

Hm. I think your grandson should be talking to someone about this, maybe a school resource person or a counselor. That he has brought this up multiple times suggests he is ruminating on it, so yes I do agree with you that he may attribute his dad’s problems to a source. As your exDIL suggested I am worried he may, in the absence of any tangible thing to connect them to (like neighbors dog barking) he might blame himself or conclude that he is doing something to cause them. So in my opinion, since you asked, and by the way, I totally understand mental health problems and all that, so trust me when I say I’m not ignorant to the reality of what your son might be going through. However, your son needs to make a further effort to lessen his “headaches”. They are causing distress is his child for goodness sake, he needs to do more instead of the status quo. He also shouldn’t get his headaches in front of his son, or otherwise have a headache around his son. Yes you can’t just turn it off but if he can’t put on a happy face for a few minutes for his son then he is harming your grandson. It seems as if your son, the father, is going about the house in such a way that it’s more than obvious he is in a bad mood, and that’s not okay. I mean, is your son out for the count for hours and hours several times a week? Why would your grandson know about this issue, why not “oh dad is working downstairs” or something like that. I hope he isn’t laying down in a common area where his son sees him and has to be quiet or anything.

I truly apologize if it seems like I’m ragging on your so Nanamar, I’m sure he is a good father and a good person (and you sound like a very caring grandmother by the way). I’m just worried about your grandson, as you are. He, your son, has more agency here than he would like to admit, so encourage him to take further steps for his child, is he actually sleeping when he is laying down? Oh, I just realized that it’s very possible that he might be playing on his phone during that time, not sleeping. If he is browsing his phone when he gets his headaches, my reply would be much different. That would be unacceptable. Hopefully that is not the case.