You'll want to make sure your guinea pig is getting enough exercise. If he's just large by nature, nothing really needs to be done. If he's overweight you're going to need to limit his food. Give him a single feeding per day of a measured amount (one quarter cup of prepared food). Add chew toys and alfalfa cubes to his play area to encourage activity. An alfalfa cube will not usually cause weight gain because the animal has to work hard to get food from it, and it will help wear down his continuously growing teeth.