I was thinking the same the Place that trained him to be a service dog should have all the rights to him and could go get him anytime they wanted.
I know that places are here if you get a service dog from them they have the right to come check on him or take him if you where to give him to someone else.

So i think the best way would be to find the people that trained him for his job and they can go get him. And give him back to the REAL owner.

I really hope he gets him back