You may be able to get some of this information without going through the formal testing process. When we were deciding if it would be worthwhile to have DS take the ACT, we bought "the Real ACT" for a pretty low price. It has 3 full ACT tests. Just from having him take one of those I have gotten a good sense of where his skills are at and where some of his gaps are.

If you have him do it on his own for your own purposes, you can have him take each section separately without any of the timing issues and test pressure. He may be a little young to take the ACT, but with his WISC where it is, I would think he would probably have scores that would register on the ACT.