acdirito...
(((((hugs)))))
sounds like an intense time for you.
Even so called healthy 18-20 year olds have to kind of make things get ugly before they move out of the house, because the separation from a parent (grandparent) can be so stressful and difficult and they don;t want it to be so painful so they kind of have to pick fights constantly etc. I would think in the case of your grandson it is doubly heard because he did lose his mother and now he is "losing" the only parents he has known.
Maybe you all need to have a group session with a counselor - so that you can lovingly cut the strings - and he can express his anger at his abandonment by his mother, and you can all move through this maturely without too much ugliness.
Hard times....
Peace-
B