I have a 9 yr. old 20 pound terrier who is extremely underweight, and is allergic to corn. We tried to feed him 2 times a day, but he won't eat. We can't regulate his diabetes his blood sugar often registers over 680, when it should be under 200 or so. What should we do?

You should easily be able to find a corn free dog chow. Your vet would be an excellent source of a recommendation. Most pet stores should carry something. Corn as a dog chow ingredient gets a bad rap. For as many dogs as there are eating corn containing chows, there aren't that many confirmed cases of dogs being allergic to it. If your dog really is allergic to corn, there are plenty of chows being touted as corn free.

As for tempting him with rich food, I think again, talk to your vet.

I have a 9 yr. old 20 pound terrier who is extremely underweight, and is allergic to corn. We tried to feed him 2 times a day, but he won't eat. We can't regulate his diabetes his blood sugar often registers over 680, when it should be under 200 or so. What should we do?
Diabetes is a very complicated disease and it can cause other organs to fail ,especially the liver and kidneys, and especially when the blood sugar is out of control. You need to get an exam and blood profile done to see what complications are going on or the pet will slowly fade away.

Mabey the food you are feeding him has corn in it also have you called a vet yet they should know what to do.:)