How long have you had him? If it's only a short time, it may be that with your current feeding, he will eventually gain weight. I wouldn't want him to have a jump in weight, personally, but a gradual one. Since Chihuahuas can be prone to obesity and it doesn't sound like he's THAT active at the moment, he won't need that many extra calories really to gain.
Xan Varik von Fernheim (born 4/3/13)