Re:
A few other things worth mentioning from own experience (I also built my own touring bike using a cheap hybrid bike as starting point):
-depending on the wheels/hubs you choose, it might condition the frame you need, as rear road hubs are 130mm wide while trekking/mtb hubs are 135mm.
-if choosing road wheels, beware of spoke count. The more, the stronger. Mtb/trekking wheels come usually with 32 or even better 36 spokes. It's hard to find that spoke count in road wheels.
-for touring, but this is personal, I find the ideal crankset gearing in the middle point, that is, trekking cranksets at 48-38-28. The standard mtb (42-32-22) is a bit too low and the triple road (usually 52-42-30 or similar) too high.
-road brakes are ok-ish in terms of braking power when fully loaded but leave hardly any space for mudguards or thick tires (32mm or higher), both a must in a do-it-all touring bike. Therefore, better get a frame with canti studs or disk mounts, both compatible with STI levers.
-bar-end shifters are ideal for touring since they work on friction at front (so it does not matter mtb or road derailleur) but unfortunately nowadays only come for 10/11 speed for the rear and only in syncro mode. Chains for 10 or higher speed are not durable enough for long distance touring. Also, they are ridiculously expensive.
Finally, in case your son changes his mind, I was also in favour of drop bars for my touring bike, and so I have it now configured (and it looks way cooler than flat bars!). However, with time and personal experience, I find flat bars more versatile for touring because:
-shifters are waaaaaay cheaper than STIs or bar-ends, no matter how many speeds.
-you can easily mix and match shifters and brake levers depending on taste or budget. With STIs, when one breaks, you need to change everything!
-bikes like to fall when you are not looking, and STIs are quite exposed. Flat bar shifters are better protected. IT is also easier to put the bike upside-down for those on-the-road repairs.
-the extra hand positions of a drop bar can also be achieved in a flat bar installing long bar ends, and grips like the Ergon make them super-comfortable.
-the advantage of an ergonomic position against wind with drop bars is negligible if you are touring with wide fully loaded panniers in a 25 kg bike.
-when putting the bike on a train, or bus, or in a box for a plane, or park together with other bikes, the flat bar can leave the bike in a much more compact size than a drop bar.
-since drop bars are narrower (42-44cm vs 60cm or bigger for a flat bar) the free space for all the gadgets travelers like to carry is greatly reduced (speedometer, handlebar pannier, GPS, bell, lights, etc.)
No matter what you do, show pictures of the final build!
