I guess the only thing you can really do is up his exercise and build muscle. Does he like to play ball? Swim (lol)? Chase a cat toy? Run up and down the stairs?

You can't physically change the structure of his body, obviously, but you can make him more "durable" by adding muscle. I would definitely not recommend upping his food intake in order to make him fatter. As he gets older he will fill out and gain more substance. Our little one, Kyra, weighed 7 pounds for the LONGEST time, and we thought that'd be her adult weight but actually gained a few more pounds and is 10 pounds at 4 years (still skinny!). She was the runt and a bottle baby as well (never did get to be the same size as her siblings, they're all about 15 pounds), but she's SO active and always playing ball, gaining muscle.

I don't understand the worry with Dora being bigger than him. Kyra at 8 weeks was only about 2-3 pounds and she played fine with 20lb Micki and Mini, 45lb Jack and 90lb Shadow. I think in your house the cats would pose as a larger threat :P I do understand that he's rather fragile at 2 pounds, but I think that's expected when getting a Chihuahua. I've read the normal range is between 2 and 4 pounds.

Here's an unusual duo at a California shelter, I just read about them and thought I'd share because it sort of pertains to this topic. It's awfully cute.
http://www.pe.com/localnews/inland/s...6.209d763.html

Or even check out PT's Bear and Betina!
http://petoftheday.com/talk/showthread.php?t=140301

Anywho, I guess what I was trying to say is that if you're worried about him you can definitely add more muscle through exercise....like mr.chiwawa said.