I would recommend getting something like this....
Old School V-Brakes, Coil fork, 6061 Alu Frame, and virtually ZERO maintenance for R1700,
and get a new Saddle, if he is not comfortable he will stop ridding... something like these...
This will enable him to go for a comfortable 30 min ride without any worries.
If he is conformable he will enjoy it, if he enjoy's it he will do it more often, if he does it more often he will get fitter, if he gets fitter he will be able to ride longer, and this will improve his health and drop his weight.
Once he has dropped some weight and "IF" he wants to take the next step, he can get a newer 29", I would recommend ...