Similar thing happened to my Grandad when he was in his 60's. He was diabetic too. First his toe, then his foot, then to just below the knee. Three years after that exactly the same with the other leg, so he ended up as a Double amputee.
When he had his first leg off, he coped really well, they gave him a training leg initially which had a crescent shaped 'foot' - almost like a D shape. After a few months of getting his balance, they progressed to a normal shaped foot.
Biggest things he found initially was the pain in the stump, but that hardened up. Oddly he could still feel his toes, apparently it is quite common, since the brain still thinks it has a foot at the end of your leg.
The second leg was a little more traumatic, he became a little depressed at losing his second leg. He had good days and bad days.
Your Grandad will need support from everyone around him. I'm fairly certain that they will do a home assessment prior to discharging him, to ensure he can get around his home and that there aren't any trip hazards. They may even end up fitting stair lifts and hand rails around his house.
My Grandad and my Grandmother were moved into a Sheltered Bungalow, that had extra wide doors for his wheelchair, pull cords in every room and a special sit in shower.
I'm not minimising the operation your Grandad had, but you will be surprised at how well he will adapt. It will be hard for all of you initially but keep smiling and I'm sure he will get through this ordeal.
By the way - you can still buy him socks and slippers for Christmas, I did for my Grandad, since he would always put socks and shoes onto his artificial legs.
