Have you had a small dog before?
We accidentally made our chihuahua fat when we first got her, because we were feeding her the amount we thought she should be eating. When the vet weighed out how much she should eat to maintain a healthy weight, I couldn't believe how little food it was. It was 1/10 of how much I had been feeding her/expecting her to eat.
I'd keep an eye on his weight, and if he's maintaining a healthy weight, the little he is eating is likely the right amount.