I wouldn't change the old fellers diet so soon after an anesthetic procedure. Feed him small amounts of his normal diet, and soften it with warm water. Hopefully pain meds were prescribed, and the most important thing you can do for him is to make sure his pain is managed for the next few days. He should be healed in 7-10 days, and feeling like his old self within a week. I've performed many, many dentals on geriatric dogs and cats and the owners reported how well the pet felt after all those rotten teeth were removed.
This is a 16 year old JRT getting a dental. He had a grade IV heart murmur and elevated kidney values. His anesthesia was adjusted to meet his needs, and was sent home with pain medication suitable for a patient with kidney disease. He had numerous extractions and was walking with in 30 minutes of completion, and walked outside to urinate one hour later. He went home that evening and was ruling the house once again at his 3 day check up. His owner was ecstatic at her pet's 30 day check up stating he was acting like a pup again!
I have more clinical pictures but they are traumatic for those not used to seeing a pet under general anesthesia.
I hope your pup does as well this one did!