I feel that petting, calm words and kindness communicate volumes to a horse or any other animal.
But he will still be wishing for his own kind.
LOL...don't be surprised if mare squeals, kicks or totally ignores him - he will probably still be delighted that she's there.
And...I think if you listen, if you're around an animal for a long time, you don't need a communicator, as their behavior and manner communicates so much. They will tell you a lot, they show you how they feel, they ask for help, all in very simple, direct ways that require only the power of observation and love (which you seem to have plenty of).
It is really sweet the way horses will be so poorly treated and then trust and love people again.
Sometimes it makes me so angry because it seems like an act of forgiveness, something I don't feel those who misuse animals deserve.
I try to think of it as just that the animal trusts that someone better will be there for them.