I think it might be easier to think in terms of prioritising between health and divorce, not your son and you husband, if that makes sense?
In a worst case scenario for each, your son will have a worse time in his divorce, and his mental health might suffer, but your husband could die.
On the other hand, you need to think about how likely it is that either situation is going to happen. If your son moves in with you, he will become part of your household, so you won't be going against the current advice (which, let's face it, could change at any time). If he can get a test, so much the better, but if not he could self isolate before moving in, so that you all know that he is not posing a risk to your husband.
When your son goes back to work, things get a bit more complicated, but again, there will be an initial risk period after which things will settle down to the same routine that so many are living with - getting changed as soon as he gets in and showering before coming into contact with his father.
The grandson moving back and forth is yet another complication (you poor thing!). Could your son see him at his ex wife's house, rather than the child coming back and forward? Again, observing all the precautions when he gets to your house, of course.
For all we know, the virus could be with us for years, and you need to get to a point where all of your family is able to live a compatible life together and separately, with as little risk as possible. I don't think it's possible to cut out all risk for everyone, but you can make it so low that it is preferable to the alternative, which is no risk from the virus itself, but high risk from the fall-out.