I would try the Stanford Binet, myself. You don't have to wait until he is six, and there is plenty of ceiling in the test. See if you can find an examiner that will let him do it on multiple days, so that you aren't tied up too long at any given time. We did three testing sessions of two hours each for my 5y3m old last fall; however, he is a VERY cooperative kid who was really engaged in the testing. (We called him the "puzzle doctor".) The psychologist, though, noted that he was particularly interested in the harder questions, and with the routing system they use, you get to skip the easier questions on most of the subtests, so you don't burn the kid's interest out answering ridiculously easy questions.
