As Jorats mentioned, he looks good in the picture
As other have said, he needs a friend such as another boy, a spayed girl or two baby boys (10 weeks old before they are physically placed with him). Intros will need to be gradual. Please see :
http://www.ratshackforum.com/forum/showthread.php?t=32368 and joinrats.com I have introed rats that were alone for most of their lives and they were so happy once they had friends, although having a large cage so that they could have alone time when they wanted it was important for some.
Please get rid of the ball - or remove both end pieces and hang n the cage as a type of hammock (put in lots of padding because their toes can be caught in the slits so it can be dangerous)
As others have said, weight loss needs to be very slow and this can be achieved with a healthy diet and lots of out time with you in a play area so he gets lots of exercise.
A good diet would be oxbow blocks (or Harlan Teklad 2014) and daily vegs such as baby kale, broccoli, cooked or raw carrots (organic taste better), cooked sweet potato, cooked squash, peas, cauliflower, mixed greens etc.
Plus occasional healthy treats such as berries and pieces of other fruit, cooked grains such as oatmeal, cooked quinoa, homemade soup, a cooked bean, a cooked chickpea, a couple of organic kumat puffs, etc
Other then the blocks try to stay away from processed foods, nothing with added sugar (or sugar substitutes), added fat, added oil, etc
As Jorats mentioned, please post a recent picture. We love pics