Please read our cat

I am just looking for some guidance and help here. Thank you so much in advance. I just don't know where to turn right now. As a pet owner, we did/do all the things you should. Spay and neuter, shots, yearly vet visits, good food, playtime, assorted perches, human interactions, and treats. The oldest cat has been in our family for 12 years, and within a year we had 4 more join. About 3 months ago, his behaviors changed. It has upset the other 4 cats in the home. It has caused mental anguish with his human. He was adopted for my adult autistic son, who is struggling with these changes of his cat "being bad". Just a bit of background, he started attacking the other cats in the home. First the female, and we keep them separate. Not ideal but it has been working to a point. Now he is attacking 2 other boys. One is missing fur, the other is so scared of his shadow, that he is now isolated with the female. When I say attack, its just that. And preys, sits constant for hours at a door, knowing they are in there. He just waits and waits. It was never like this. All 5 cats have been together for 11 years. Tonight I had to switch living arrangements and put him into isolation and let the others free roam. It will take them a few days to figure out the big guy is not free to mess with them.

All the suggestions we got have not worked. We went to our usual vet and a different one in the area for a second opinion. We have done several tests with the vets (blood work, ect). We even used suggestions from animal rescue groups about behavior corrections, and CBD oils and powder on the food for vitamins and immune health. I need to know what is wrong. What is going on with him? Does he not love his home anymore? Did we do something to him? Is he sick? Am I not asking the vet the right questions or is the vet missing something? What do you see? What message is coming through that can help us help him.