Ok I see. Most people when they say their dog is too thin think that a dog that is a 5/9 or 4/9 (which is ideal) is too thin, but yes a 1 or 2 is too thin. Especially for a Chihuahua. You may want to consider a puppy food for a short time to get him back to a 4 or 5 out of 9 and then try to maintain that. Edit: Sorry I see you do have him on a puppy food, so yes keep him on that.
I am a big fan of Purina Pro Plan and fed the Sensitive Skin & Stomach formula for about 1 1/2 years to both my dogs. If it has the right amount of calories your dog needs I would recommend it. I will caution you though that the pieces are probably too big for a tiny dog like that.
