If your herd animal has always lived alone, with you his only company, now he finally finds himself with a live-in friend, chances are he's afraid of letting you catch him & take him away from that. So if that's the case, it will be a matter of proving to him you're not going to do that. If you can, section off a corner of the paddock, to help you catch him, then when you do, do something nice & let him go, without taking him from his buddy. Making friends with his buddy also helps an 'uncatchable' horse learn to trust you too IME
Sometimes horses who have been kept away from other horses do have social problems that cause it to be too difficult for them to live in company, but this is not generally a big problem. Perhaps the other horse is being 'mean' to your boy because your horse hasn't learned the finer bodylanguage messages. Or perhaps he's ignorring or rebelling against this horse's authority, or the horse is otherwise feeling insecure so it's 'upping the ante'. Or perhaps it's just normal 'horse play' that you're not used to, having kept your horse alone till now.