It all sounds good. If he is up to it, it is okay if he wants to eat an entire pie. It will take weeks or months to put the weight back on. If he wants a milkshake, enjoy!
The fatigue should begin to go away soon however if it does not, get his Thyroid checked. Radiation can damage the thyroid sometimes. If his TSH is high or low, this scan lead to overwhelming fatigue. Or he may and probably is just tired whilst healing.
Get him walking, even if its just to the street and back. Not moving is not allowed. I don't know where you live but I will be happy to come there and kick his butt. A lot of work went into curing him. Now he has to do his part. This is said with love by the way, in case he asks.
Courage.