Many moons ago I placed exchange students from Germany with families in England.
It's pointless talking to a different exchange agency, as he has paid his fees to the one he is placed with. You need to talk to them. They should also have access to German speakers so he can accurately explain his situation.
It shouldn't take much effort to be approved yourself to host him, and you should be paid a per diem to cover some of the expenses of feeding him.
Just give the agency a call and let them know what's going on.