I'll recommend the Cyclist training bible again as it is the best training book written to date.
I sense an appropriate birthday present for the boy.
Thanks, Gonzo, you've echoed what I've been telling him. Basically, get lots of gentle miles in your legs, and don't go out thrashing yourself all the time.
Not gentle miles, time. What you are doing is getting the muscles used to working continuously for a period of time.
<cod physiology follows>
Look at how the body burns fuel. For the first 10 seconds it is pure ATP. For the next 10 minutes it is available glucose, then glycogen breakdown to glucose for the next 45-60 mins. Then it becomes a case of being able to replenish that glycogen which you do either by eating or by breaking down stores of fat. Both are possible but you cannot break down fat fast enough to replenish glycogen at even a moderate pace.
To enable delivery of glucose to the muscles you need to put a strain on the body so it responds by increasing the blood supply. It does this in two ways, increasing the pulse rate and increasing the blood vessels feeding the muscles. Developing these takes time (and also explains why a seasoned racer can take a year off, be really unfit, and then be back to race pace in a sickeningly short space of time- they already have the apparatus.) - quite literally. You need to ride far enough to put a suitable strain on the legs (at least 90 mins, preferably longer) and over a period of time. The other advantage of the improved blood supply is better lactate clearance, which means you can go harder for longer.
Long easy miles will not do this. You need to feel that you are putting in some effort (though not necesarily much so level 2 rather than 1. Pushing too hard will get you into lactate hell or hunger knock. A brisk ride of a couple of hours to a cafe and back without too long a faff would be good to work up to - even better are rendezvous rides where he can take a longer route than the slow potterers and aim to get there before them.
As variety, if he can only find time for short rides, then make them as hard as possible to build strength and speed (the long rides will build endurance).
There are lots of things to work on, and they all work in different ways. Setting personal challenges such as distance travelled are always good. Even better if you have a shortish loop that can be ridden when he gets back from a ride to just tag on those few extra miles.
..d