Can you confine him to a bathroom with the litter box for a few days. I have heard folks that have had problems use this method to get the cat to use the box again. I think it works. At least it would confine his inappropriate elimination to one space.
There is also a litter called Cat Attract Litter that is supposed to work for enticing the cat to the litter box.
There is also a hormone spray this is supposed to calm cats called Feliway (I think) it comes in a spray and a plug-in. It supposed to calm cats so they are not so tense around each other.
Good luck.