Can you see the hip bones and a loss of muscle mass as well? If so you may want to give your vet a call and talk to them about it. If it is just a weight thing then just a high quality diet and regular feeding (several small meals a day if they've been starved or weren't eating for some reason like recent illness) will quickly put weight back on in a healthy way, nothing else really needed, unless maybe a bit of some canned daily. Too much too soon is unhealthy weight gain and a bad way to go about it. If the pup does not gain a little weight within a week or so of regular eating and some canned food you would want to consult your vet. If the dog has been eating and is under weight or has been losing weight in spite of eating it indicates an underlying health issue and you would need to see the vet to find the cause and correct that and some have guidelines for feeding as well (like kidneys, ect.). Here is the analysis site to check out high quality diets. I feed Innova, green bag for a comparison point (and also a recommendation since it's awesome, lol). Hope that baby is all back to normal soon.
www.dogfoodanalysis.com
www.bornfreeusa.org
www.terrificpets.com
www.chihuahuaclubofamerica.com
www.dogfoodanalysis.com
www.bornfreeusa.org
www.terrificpets.com
www.chihuahuaclubofamerica.com