I'm really happy you were able to get it on him and it's working. The pain will continue to ease up until it is gone but it really is important that he tuff it out and wear it when he is sleeping. I have the same problem with being overweight and it digging into me so I make it really big (if I hold it against my back I can fit my fist into the front)and this seems to help. If he wears it all the time he will get to the point where he will not even notice that he has it on. The extra weight he has gained is probably what he is eating and not getting any exercise, not how much he eats.
Georges Lakhovsky who is the person who first found out the benefits of the copper coil put them on animals first. While he usually put only one on he did add them to the legs of horses. He would put one around the neck and one around each leg. But I honestly believe that it won't be necessary if you can get him to wear it all the time. The other guy that put it on and was in horrible pain for 5 years was relieved and normal within 3 days.
Remember above all it is important to keep it clean using a copper cleaner. I have to clean mine every day or two but with the pain he's in I would clean it every day.
Good luck and let me know how it's going. Did you read the message from "Jackdaw"? He made one up and is getting relief also.