Our 3 year old dog eats our puppy's food?

We have a 3 year old terrier mix and a 6 month old lab. The pup eats Authority large-breed puppy food, while the terrier has been refusing his own food and eating the puppy food for the past 3 months or so (we've never had a problem with him eating before). I realize that him eating puppy food means extra calories, but didn't think anything of it as he's pretty hyper and runs a lot. Well lately we've noticed him itching intensely, his skin is pink all over (more than normal) and he has developed little sores on his skin. (Neither have fleas - they're both on Advantage as a preventative). My husband thinks the skin problem is from the extra protein in the puppy food - could that be it? We have tried separating them by feeding them in separate areas of the house and having the older dog go back to his regular food. He will finally eat it when he's hungry enough, but the itching hasn't seemed to stop much. We haven't changed anything in the house to where we can pinpoint an allergy source other than the dog food. Wanted to try and figure this out on our own before going to the vet...any suggestions?