It sounds like this has been going on for a long time, so it may be difficult to break this habit, but I think it can be done.
The room needs to be completely cleaned, saturated, with a special enzymatic cleaner made specificlly to remove pet odors. Aloicious can still smell that the floor in that room has been used before, so to him, it is okay to continue to go there.
Where do you keep his litter boxes now? How many do you have?
Keep the door to the room he's been soiling in closed, so he can't get in there.
Give him at least two litter boxes. Why did you get an enclosed box? Many cats do not like enclosed boxes.
Use Cat Attract litter. While you are cleaning the room would be a good time to do the retraining.
For retraining, the cat is kept for two weeks in a very small room (or a large dog crate) with only enough area for his litter boxes, food and water dishes and a bed. He will be forced to use the litter boxes, as he will not want to soil his bed or feeding area.
You might want to try installing some feliway plug in diffusers too.
I'm glad you brought him to the vet and had the blood work and all done. That's important at his age.
Welcome to the forum, please keep us updated on how things are going.
PS I'm sorry for your loss of your other kitty.