After the surgery he will be on tiny liquid only meals for at least a week.
He will feel exhausted and ill for many weeks, simply because he cannot eat as much as his body is used to.
Then he will be on tiny portions of bland food for months. He will feel nauseous and actually vomit. Food has to be very well chewed so it is actually unpleasant in the mouth.
In the long term, he will never be able to enjoy a 'normal' meal. Eating out will not be the enjoyable family occasion that it is now, nor will special meals at home.
No fish and chips with the grandkids at the seaside. No anniversay celebratory meal. It is all very grim.
This is assuming of course that all goes well. Any operation is risky - and the risk is greater for obese people. The operation could go well but later complications could set in.
Over time your dad could cheat. People do. Melt chocolate to make it more easily digestible. Eat small poritions - but of the wrong things, and lots of them! Some people still end up larger than they were before the operation after a few years.
If your father really thinks he can handle all of this, not just for weeks or months, but for the rest of his life, - why can't he just diet and exercise?
I wish him well.