A warning first: I'm an engineer, with some work and studies in Ergonomics, but I'm not a physician. I am also a bicycle tourist, often with two children. And I don't have direct experience with single-handed cycling.
I don't see any problem in using a bike with road bars if that's what your son wants. In fact, as a long-time cyclotourist and user of drop bars, I would prefer to ride single-handed on a bike with drop bars rather than on a bike with straight bars. I think part of the solution is to raise the bars so that he doesn't lean too much.
My first question is: does he uses and puts weight on his right arm? If that's the case, I would look either for:
– some kind of receiver for his arm: a sort of prosthetic end into which he would put his arm, or
– an aero bar towards the right side, so he could lean his right arm and elbow on it. I think it's possible to modify an aerobar to support his arm in a higher position, but I'm not an expert on this issue. Ideally, he should be able to use the drops with both shoulders at the same level.
If he is able to control the bike effectively with both arms/hands and keep it in control with his right hand, it opens a lot of possibilities for shifting and gearing.
Stem and handlebars
I would favour relatively high handlebars. Look for an uncut fork, fork extender (heads-up stem) and/or a stem with lots of raise. That way, he won't have too much weight on the arms. It will always be possible – and cheap – to lower the handlebars if/when he wants it.
Brakes
With cantis or road brakes, you could fairly easily install two brake levers on the same side.
For a while, I equipped my tandem with a drum brake (+ 2 rim brakes), so I needed a way to operate three brakes. So I installed
two brake levers on the left handlebar.
The two main levers are unmodified Dia-Compe 287-V levers. The third one (on the left) is installed slightly lower and turned inside. It's a basic Shimano Exxage left lever, and I had to cut a bit of the rubber hood and file a bit of the lever (the lower lip only) to make it fit. My hands are just slightly longer than average, but I can even grab both brake levers at once without too much effort. At 11, I'm sure he would be able to easily use either of those levers from the drops and maybe both at the same time (it will come soon anyway). And he would be able to use the top lever from the tops or hoods.
Having two brake levers is the ideal solution in terms of control, especially if he rides a bit off road, on loose sand, gravel, ice... He could use the front brake most of the time, but the rear one if it's slippery, icy, etc.
Which lever should control the front brake? My hand falls more naturally on the lower lever, so that's the one I would use, but I know most people reach more naturally the upper lever. I am also a bit at odds with modern trends in that my bars are level with my saddle and I ride on the drops 80-90% of the time and
never on the hoods.
Another solution has been covered already: use one lever to control both brakes.
As far as I know, only one lever allows that, the
Dia Compe 287 T, which is available at tandem shops (ex. Tandems East, Precision Tandems, Harris Cyclery... Visit
the Tandem Link for addresses in your "neighbourhood"). One slight problem: it's the right lever that is made for dual cables. However, I just looked at my 287 V lever which is designed fairly similarly, and it would be fairly easy to file other cable-housing notches to allow installation of the dual lever on the
left. Another problem: the 287 T levers are not cheap: 60-100 $ a pair, whereas basic Shimano levers are 10-20 $ a pair. And many women don't like to operate them with their small hands.
And yet another solution would be to use any brake lever you want and a cable splitter. J-Teck Engineering makes one. If you put the splitter far on the line, it would allow you to add an auxiliary brake lever.
I'm not sure how I feel about that, but installing an auxiliary brake lever (also called Cyclocross lever) might be a good idea. Basically, it's an extra lever that's installed on the flat part of the bar and allows braking from the top. Downside is that it crowds even more the left side. Whether he uses it or not will depend, I think, on how well he supports himself with his right arm and whether he prefers to ride with his left hand on the drops, on the tops or on the hoods.
You could always install that lever later and decide onto which brake it should be. If you use a single brake lever + splitter, you could even install it beforehand, so the lever controls both brakes.
Shifters:
The Rohloff hub is indeed a very nice option, but neither lightweight nor exactly flexible if he wants to finetune the ratios. For single-handed operation, I would either install it somewhat like the Kelly Take off (see below, and you would have to adapt it, because the Take Off is not compatible with the Rohloff shifter). I also remember seeing a bar-end adaptor similar to a gripsift. EIther at
http://hubbub.com or at
http://sjscycles.com. Worth exploring if you have the money.
Otherwise, I see a combination of 2-3 options.
- My preferred option would be to use a
right bar-end shifter on the left side. The only minor issue is aesthetics: you'll have the knob that selects "friction-index" on the outside, rather than on the inside. But it will work without problem, he won't have to bend his wrist unnaturally...
When I ride on the drops, I keep my hands on the drops and have no problem shifting up (ex.: 1 to 2...) by pushing the lever down with the palm of my hand. I downshift the same way by pulling it up with my little finger. I suspect he will have to grab the lever in his hand to downshift... but that's what most people do anyway.
- The front derailleur could either be operated by a regular left STI lever or by a downtube/bar-end lever on a
Kelly Take Off. Kelly manufactures a few models, including one that allows two levers on the same side. It might be a good solution, although many people find that they have to move their hand too much to operate the lever.
P.S. If you are friend with your bike shop, you might find a used left STI lever. They sometimes break and some people upgrade their system, and in both cases, it's the right lever that goes down the drain.