Hi,
I have got a (much smaller) mbu, he tends to confine himself to the right hand side of the tank also, however I've noticed that he isn't keen on the bubbles that are produced from the Ario that I have and he doesn't seem to like swimming in them. Well that's not strictly true, he did like it for a short while and he'd take a mouthful of the bubbles and burp them back out again - very strange behaviour. Anyway, I've turned the bubbles down a bit and thats made a difference.
I've also noticed that he swims in more of the tank when he is hungry, I presume that he is scanning for food.
How long has he been like this for? If nothing is showing on your water test, the only thing I can suggest is to take a sample of your water to a local aquatics expert for further testing who may be able to detect a trace of something that the home tests can't. Perhaps someone else on this forum may be able to comment on this further, but it's something that I've always done in the past.
If he is his usual colour, is eating ok and still seems active in his corner, then the only think I can think of is that he doesn't like something about the otherside of the tank - have you tried moving plants or objects around to see if that makes a difference?
let me know how he gets on - very handsome puffer by the way.