Appy rider you just explained my horse to a t. He is unbelievably buddy sour. He'll be in a turnout with a new horse for an hour and they're attached at the hip. I can't tell you how many cross ties he's broken in his crazed panic to get back to his boyfriend. It's fixable but it takes time.
First of all, is there anyway he can be turned out alone? If there is, I'd say thats your best route. If he's lives alone (but he can see other horses), he'll naturally get more independant and get used to being by himself. If he can't be turned out alone, just keep at it. Take him out everyday, even if it's just to handwalk him for an hour. If you can, take his pasturemates out everday for a while so he gets used to both being alone in a turnout and while working. Eventually he will realize it's not the end of the world. With my horse, I do a lot of transitions, backing up, figure eights, bending, anything I can to get his mind off of being away from his pasturemate. He gets worked. By the time we get back to the barn he couldn't care less about where his friend is, he's so tired! Just give it a try, I know it's beyond frusterating but worth it in the end.