My 11 year old Brittany Spaniel disloacted his jaw about 7 weeks ago. The vet popped it back in place but informed us that he should no longer eat dry dog food because the crunching could cause it to pop out again. We began a slow shift between Purina Dog Chow to Purina soft Alpo Prime cuts. De seems to be doing alright as far as his digestive tract is doing. The problem is that my dog is losing weight. He was 39 lbs before and is now more likely around 30 lbs. Also, his skull is more pronounced and appears to be mis-shapened. We increased the amount to food and thus slowed down his weight loss quite a bit. We feed him about 14 oz of food, twice a day. Somehow, it doesn't appear that he is getting enough nutrition. Does any body know if that it enough food based on dog breed and weight? (He should be 35 lbs ideal)

take him back to the vet, if his head is misshapen, he may have reinjured his jaw. 28 oz of food should be more than sufficent for him

Some animals dont adjust to food changes very well. I'm guessing that he will get back the weight slowly, but surely. Did the vet mention anything about him being overweight, or that he will have side affects to the food? I think that you should call your vet with this question. He can probably answer over the phone, or at least one of the assistants can. Has your dog been acting like hes hungry [like looking in his food bowl multiple times a day when there is no food in it, begging for food, etc.]? Ask yourself this. He might just be getting the affects of old age.

You should get bloodwork run on your dog to see if there is not another reason why he is losing weight.

First, you're feeding him crappy food. You need to switch to a decent brand, nothing from the grocery store or Petsmart. I suggest:
Artemis, Merrick, Evangers, Wysong, Wellness, California Natural, Health Extension, Chicken Soup, Solid Gold, etc.

I would feed 2-3 large cans per day to help him gain weight, then 2 cans or so a day to maintain weight. Also, make sure he's getting enough fluids, sunken features are a sign of dehydration.