Thinking about my approach, I do not "carb load" as such but do consume predominantly rice or pasta most evening meals particularly on tours where I am often days between 'leisure stop[s]'.
My tours are normally time limited due to home commitments, and/or prior booking of buses or trains (we are limited in the number of bikes to be carried on any one service so on popular destinations/departure points booking is advisable).

Tell me about it. I take blood thinners for my mechanical heart valves. They are balanced by Vitamin K, which is contained in many green leafy vegetables. Finding those is sometimes a challenge.

When I think of "carb loading", like you described with your son, I think of events that last at most for a day. I specifically think of the few times I tried "carb loading" decades ago when I was in some cross country ski races that were 55km long. Then we tried to do a carb depletion phase, followed by a couple days of high carb intake that minimized fats and proteins. In my case it never worked, but my training was always inadequate so I can't really say if it does work for a competitive athlete. I was not a competitive athlete.

I do not know how long an ultra marathon as your son described it is, maybe it lasts more than a day? I think it is impractical to try to boost your energy for the days or weeks of bike touring by adjusting your diet for the days before such a trip. That was why I provided a link for an article on the fuel types your body uses at different energy levels (touring is not marathon racing) and a couple links for multi-day through hikers, like those that through hike some trails in USA that are thousands of km long, such hikes take months.

I think the through hiker comparison instead of the ultra marathon racer is a better comparison for nutritional requirements for bike touring. But bike touring is somewhat different from through hiking in part because the weight of food is on our wheels, not on our feet, I often buy canned foods for bike touring but I would never consider carrying that kind of weight for backpacking.