Take him to shows, trail rides, different stables anything to get him away from his buddy so that they arnt so attached. I suggest lunging him either before you take him over to the show grouns or after you get there in a warm up ring. He needs to keep focused and getting a little energy out wont matter. I also suggest that the oowner of the other horse takes him off the premises so that your horse gets that he can go alone and it wont hurt him.
Hope this helps!!
Horses are my life, if theyre wasnt such a thing life would suck!