Maybe, Jackson is living the way he wants to live right now. Perhaps he is appreciating the other two dogs who you have said he has known a long time. Dogs are pack animals, and these dogs have likely become his pack and he feels part of their family.
Take Jackson for walks, and maybe include one of the other dogs with you on the walk. Take Jackson outside to play fetch and if you need to bring the others, then do so.
At age of nine, Jackson must feel he missed this chance to be with the other dogs, and likes it. You are lucky he isn't moping around, but is happy.
If you feed Jackson, this may help, but I would not expect him to give up his friends. Just include them, like you would with a child