Viscous Lidocaine may also help on the worst spots - ask the doctor about that.

As long as you dad is getting calories and liquid in, meal replacement shakes are fine for the short term. My husband was still on a liquid diet 6 weeks out, but we were experimenting with home-made protein shakes that tasted less bad than the canned variety. I am guessing that you could come up with a simple recipe that would be ok with the diabetes. I made packets of ingredients for my husband to take to work to add to milk and (eventually) canned fruit in the departmental blender. Canned peaches were the mildest. Whey powder, Benefiber (if he is still on narcotic pain killers), sugar free Carnation Breakfast Essentials could be added to milk. He should have the services of a nutritionist available - that would be another source for a diabetic-friendly formulation.
Best wishes to you and your Dad - slow and steady wins the race.
Maria