A tablespoon a day is all they need. Pellets are the least important part of a piggy’s diet, making up only 5%. They’re the chocolate of the piggy world. Hay, hay and more hay is the way they will put on weight. Just bear in mind that weight will be gained slowly.
How old is your piggy and how much does he weigh? You can’t go on weight alone - heft is the best indicator of condition as there is no ‘set suitable’ weight. If there is a downward trend (at each weekly weigh in) then it’s okay. Fluctuations of up to 50g a week are ‘okay’.
The less pellets he has the more hay he will eat. It may be hard at first but just drop him to a tablespoon. You could scatter feed to make it more fun for him.
Weight - Monitoring and Management