Thankyou for the responses!

I bought some high-calorie dog treats for him today. I think maybe he's kind of like all my kids were - they'd plump up right before a growth spurt then get really skinny.

I'd always heard chihuahuas had the opposite problem!

This country life could be doing it, too. Max has lost SEVENTEEN POUNDS since we moved out here! He just runs and runs. And the chihuahuas spend a lot of time wresting in the front yard, whereas before they were confined to the house most of the time.

I'll post some pics soon.