I think it's possible for kids that young to be able to keep them well, but obviously not all kids are going to be capable. I think it would depend a lot on how responsible he is, how dedicated he is, and of course learning the proper husbandry. I got a veiled when I was 7 and it lived for a long time. I, however, was unusual in that I was really dedicated to things that I knew I was responsible for. I read a few books on chameleons and did nearly all of the care-taking myself (my mom had to buy the bugs/equipment and drive me to the vets when it was necessary though. You can't do that by yourself when you're 7!)
I would say, don't unless he is really, really interested in one, hasn't shown a likeliness to grow out of his "favorite" things (ie, only likes something for a month or two and then starts to neglect/hate it,) and is really responsible. Also be prepared to do most of the work or all of the work if he decides that he doesn't want/like it anymore or just doesn't have the ability to give it the care it needs. Some kids might be ultra dedicated but just lack the capability to give the animal the care it needs.
I think a crested would be a good starter reptile though! Plus they can be taken out and handled so they would be more likely to hold a child's interest.
