One of the big challenges post challenge will be keeping his appetite under control. That's going to take some significant discipline or major weight gain unless he can keep the riding going.
From a personal point of view, I'm not sure this is a problem.
I did about 11,200 miles in 1994 & 1995 and mostly weighed 72-76kg then
I did around half that in 1996 and was around 72kg.
I rode very little in 1997 and lost weight without dieting, due to stress of being diagnosed with MS but was never bedbound.
I gave up cycling around 2002, weighing around 70kg and stayed at about that weight until I acquired a croissant habit following a trip to France in 2009.
My weight then crept up to 74kg at the end of 2011 and I've been battling with fat since then, and winning slowly.
I am almost totally inactive and don't have the voracious appetite I had when I was riding and doing a full-time job.