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Cash, credit card, rain gear, extra quick-dry clothes that can be washed out in the hotel sink or laundry. Spare tire or tube, puncture kit, pump, bike-specific multi-tool. I know someone who toured France this way, very minimal gear, stayed at hotels or hostels every night, ate out every meal. You want enough pack space to carry food you pick up at stores along the way, so you can have a picnic lunch.
>overweight, 66 years old
I'd suggest a training regime first, maybe try a 10 mile ride, then 15, then 20, etc. I knew people in good shape in their 60s-70s who could handle century rides, 100 miles on a day. Make sure the bike is in good shape, too. Find the best mechanic in town and have it gone over.
A lot depends on the chosen route, how much civilization is on the way, how often he can stop for the night. Sounds like a fun idea.