with HD especially, you want to keep him as lean as possible, weight on those hips is not good
Now 80lbs may be good for him depending on his structure, but he may stand to lose a few. Can't tell you without seeing a pic probably.
Anyhow, mine would eat a 40# bag of food and have always been hogs
I limit my adults food to 2x a day, 2cups each meal, they get some treats in between, a biscuit here and there type of treat.
How much is eating when you feed?
Good luck with the surgery..Swimming is a good way to build/keep up muscle tone and low impact