If he is put in a separate pasture than all of the others, he paces the fence line where the other horses are.
Some horses are very attached to their stable or herd mates.
This sounds like that could be the case in this gelding. At 14 you are not going to change his personality drastically so you will have to decide if his behavior is going to interfere with your enjoyment of him. If you always ride in a group and never alone and plan on keeping him at a barn with a group of other horses you should be able to deal with it. But if you are going to ride out by yourself at all, or keep him at home on his own he will not be happy. You do not mention what you plan to do with him, or whether his herd sour attitude affects his work under saddle.