I"m sorry for his situation. Please tell him that he and girlfriend should just stop trying to figure out her intent. They cannot read her mind and should not waste time doing so.
When you say "a year and a half of separation," do you mean actual LEGAL separation in a court of law? Because if not -- it is past time for him to lawyer up. Yes, it will cost him. But he needs to find the money, or find a lawyer who does payment plans. It is time to make some things formal here.
As Suzanne rightly says below: I'd tell him to get her to let him in one last time (don't tell her it's the last time), when she is not going to be around, and haul ALL his stuff out. No more bits-and-pieces trips back to the house.
Then he presents her with divorce papers. His son is old enough to tell a court what son wants regarding where he lives and how he visits both parents.
He also needs to stop, as you put it, "trying to be nice cause he sees it as the path to his son and his stuff." No. The path to his son is a full and formal custody agreement detailed in court documents. etc. His property should be dealt with in the divorce.
He cannot fully commit to the new girlfriend until his wife is out of his life in this way and only in his life as his ex-wife with whom he has a custody agreement, period.
And frankly, a divorce would be good for HER in the long run. As long as they are not divorced she may feel she can keep him on a string, and she knows that as long as they are legally married, he cannot move on with his life, marry the girlfriend if he wants, etc. She needs the reality check of a divorce, which might help her realize she can't yank around another adult this way and might help her move on in her own relationship.
Please update us here.