How much does he weigh? Smudge is a skinny guinea pig, and Patches is a bit pudgy. Any different changes of brands of food? Or a new environment, etc.? Changes can temporarily stop them from eating. Also, how old is he? Did you recently get him? Is he the only guinea pig? I'm sorry, this is a lot of questions but they help to figure out if he is a healthy weight. I give my pigs 1/2 a cup of pellets and enough timothy hay to feed their hunger, not that I'm saying you're giving them a bad amount of food, though. He may not need to gain weight at all, as he may be a very healthy weight. Male guinea pig's weight is usually around 2 to 2 1/2 lbs. So weigh him and basically monitor him to see if he is eating.