Is my 4 month old chin underweight?

[Aheart04]

I've had Gandalf a black velvet chinchilla for two months now, and I'm worried that something is wrong...he hasn't gained any weight and weighs only 220g. Is this okay? I know it's low, but should he be gaining more?

 

[Chinmama]

What are you feeding him, and does he have a wheel in his cage?