What type of food are you feeding him at the moment? And how much?
Since he's 6-8 months old, he can be transitioned onto a timothy based pellet if he's not already on one. Or if timothy pellets are difficult for you to get in your area, go for a low protein alfalfa based pellet (12-14% protein). And Little Bay Poo is correct, about 1/8th to 1/4th cup is all a small bunny needs, and you can go even less with about a table spoon if a bunny is more prone to weight gain.
Is your bunny getting hay? And if so, what kind(s)? Unlimited grass hay should be available at all times for a bunny.
All changes to a bunny's diet should be done gradually. So if you are changing pellets, or the amount of pellets, it should be done slowly over a couple of weeks.
--Dawn