Some interesting points of views on this thread, none are wrong but interesting none the less.
I own both a 2yr old mac book (not a pro and not an air) and an iPad and they cross over in their uses in some respects but each excel in different areas.
Instead of carrying books around little beats the iPad, for manuals such as software dev ones the screen presentation is better than that of the macbook and the kindle with navigation, note taking and thesaurus well thought out and a pleasure to use. However no software development can take place on the iPad as there are no compilers available and apple will not let there be one due to the security sandbox that applications run in.
For simple document editing you can do this on the iPad but one constraint is the keyboard, however a bluetooth keyboard such as provided by apple works well with the iPad (and the iPhone & Mac Books) and the one I am using is an apple imitation from Amazon that cost a whole 11 pounds.
I am writing this on my MacBook (using said keyboard) along with a belkin bluetooth mouse. I have an external monitor and these along with a second charger at my workstation in the living room, so I get my laptop out when home - plug the monitor and charger in and I am ready to go in twin screen mac heaven.
It really depends what your son will want the device for, but like most I would err on the side of the Mac Book purely because it gives you more options; most notably of which is that if required you can use vmware fusion (my choice) or parallels to run a windows installation alongside os-x on the mac. You then get the best of the apple and windows world on the same device.
Ultimately for the cost of a macbook air you could purchase a second hand macbook and a low end iPad so you do have plenty of options. The two real old macbook that I am running runs lion well as well as windows 7 at the same time (the only upgrade I have done is put in an extra 2gb or ram to take it to 4).
I think probably the best way to look at the devices is that the iPads are for the consumers of creations and the laptops are for the creators.
Best of luck
J