E.,
I know the scene well. I stayed at home with my son all his life. But, I needed a break too, with his father. (date nights). I know the struggle that goes on, but if you trust the person you are leaving your child to, the struggle only lasts a few minutes after you have closed the door.
My parents were not local, but visited as often as they could and we would visit them as well. I found that when they came to our home, my son would cling and refuse interaction with them. If I ran an errand shortly after their arrival, a trip to a convience store for example, when I got back after just five minutes, he had relaxed and was playing and interacting. The rest of the visit would go well and I would have extra hands with the children during the visit. If I did not do this, it would take two days and the visit would be almost over.
You are attentive, and bonded, if you trust your family, get a quick break, and show your child that he can trust family too. Don't allow them to take it personally when he chooses not to interact, he gets to interact on his terms. Just try the quick removal of yourself, out of the house completely for a few minutes, and observe before re-entering. You may find that he clings and cries until you are gone, plays and interacts while you are gone and then cries again when you return. Doing a quick glance before re-entering helps the resolve.