I just have to say, please listen to these people.
Right now I'm working with an orphan (now a 10yo mare) who was "messed up" by the people who previously had her. They coddled her, didn't let her around other horses, and let her get away with murder because it was "cute".
Well, lemme tell you, "cute" isn't so cute anymore when the "baby" weighs 800lbs.
It sounds like you're gonna do the right thing by him which I'm super glad about. Definitely find/borrow/whatever an older mare or even a really laid back gelding. At this point, I'd say that anything that won't seriously injure him is better than nothing.
You say you have three other geldings, did you introduce him to them all at once or one at a time? If you did a 3:1 intro, you might have better luck choosing the most laid back gelding and introducing him, alone, to the baby. If they work out, the baby'll have a "protector" for when/if you introduce him to the rest of the herd, if that makes sense.
Thank you for trying to give this baby a better life, in any case. :)