Are you willing to try medication?? If so ask your vet about what might help him relax around the other cats, and maybe he can learn with time to tolerate them.

You could try a Feliway product, also. The plug in (if an outlet is available) or the spray or this collar for him to wear.
http://www.thecatandkittenstore.com/...eliway,/Detail

No guarrentees, of course, and you could continueto attempt to rehome him.