Help with finding my horse a companion?

It is not really that he needs a "playmate". He just may need, like you said, a companion. Probably any choice would be good. Since he is so versatile and friendly, he sounds like he would be accepting of any horse you would bring. If he can touch horses over the fence, that is good for him that he can do that at least. He has some horse buddies and that is great for him. You are very considerate and kind to want to get him some company in his paddock. I wish more people would have that kind of understanding and caring about their horses.