Kell, there is a pheramone you can get from vets called Feliway, you can get a plug in, and that way they can relax a bit. You can also buy it in a spray and spray the other cats... that way they are a point of relaxation for him.

If he's put on a lot of weight, there could be an issue like thyroid that is going on. Often thyroid issues can cause weight gain and behavioural changes. Its worth getting checked.... HTH