Hi & Welcome!
There are a few guinea pig diet staples you should know of. One of those would be unlimited timothy hay, oxbow and kleenmama's (ordered online) are the best. Another part would be pellets. You pig should be getting 1/8th a cup of quality pellets. For pigs over 6 months Cavy Cuisine is recommented, or again Kleenmama's. Do not give your piggy anything with colored bits or seeds, they can choke on them. Another thing would be veggies. Your piggie should get about a cup of veggies a day. Since they cannot produce their own vitamin C it's a good idea to give 1/8th of a bell pepper a day, & cilantro. Another staple veggie is romaine, but you can find more information about that in the feeding section.
Do you have a way to weigh your pig? Some pigs are bigger than others, but if you have a weight and a picture it might be easier to tell wether or not he's overweight. As far as exercise goes, never use a ball! I'm not sure if you know this or not, but they can break or very badly damage their backs, feets, and/or legs. So, don't do that... A good thing for exercize would be out of the cage/floor time. Pick an area of space, block it off so your guinea pig cannot escape, and fill it with hides, homemade toys, or whatever you have laying around. The first couple of times your guinea pig may be a little scared to explore, so once it's used to veggies you could place veggies around to encourage him to move around.
Can I ask what kind of cage he has?
Also, please post a picture of your boy!
Welcome again, if you have any questions feel free to ask.