I've thought the same thing about my horse before. If we ride in the indoor arena when all the horses are outside, he calls to them and sometimes acts up. Same thing goes for riding outside when all the horses are in their stalls. Also, whenever we get near any horse, he loves to stop, sniff noses and socialize until I pull him away.
Is your horse very social like this too?
Our turn-out arrangements were moved around a bit this summer, and for a couple weeks, my horse was alone in his pasture. He seemed so depressed! Then, after 1 day of being out with another horse, he perked back up to his usual happy self.
If your boy is super social, I don't think I'd recommend separating him.
Maybe try to work with him on seeing YOU as part of his herd. I did a lot of groundwork with my horse...taking him away from the other horses...taking him to where he can't see the other horses, etc. All the while, talking to him, treating him, having him succeed at simple tasks that I asked him to do, praising him. Now, he is perfectly calm and well-mannered when I walk him anywhere. Our next step (we're working on it) is to get him to behave alone while I'm mounted. I think it's harder for him because he can't see me. I talk to him a lot and reassure him that I'm still there with him.
Again, I don't think solitary confinement is the ticket. Build his confidence in you as part of his herd.
If you really need him to be able to be alone (w/o you riding/working with him), you could work with having him be alone for short amounts of time when he's occupied and happy...such as feeding time. Put him in his stall alone with his feed, then just when you start to feel him get nervous, bring in the other horses for feeding. Each day extend his alone time by a minute.