You may have discussed this on another post but I missed it – is Baxter neutered? If not, I’m sure that others have already told you how much that helps with the litter training.
As far as the couch, it seems that Baxter thinks that the under the couch area is now his “territory” and he is claiming it and marking it as his. I think that really the only option is to block him off from getting under the couch. There are multiple ways to do this, you could cut some cardboard to the height it needs to be and use that to block it, or create a fence of “neat idea cubes” (those square grids), or you can come up with your own creative ways to block this area off. In my opinion, that is probably the only way to really guarantee he doesn’t pee under there anymore.
Is he peeing/pooping in his litterbox at all? and is he peeing/pooping anywhere outside his litter box other than under the couch? If it is just under the couch, it sounds like he might be better litter trained than you thought and that he is just marking his territory.