Sure. It's fine. The other common option is the CAT (aka the California).
Do be aware that if you're not teaching to the test, there may be some (small) holes, especially in social studies and maybe even science, despite the fact that I KNOW your son is a science whiz. But if you never talked about something that's on the test--like, say, that a maple tree comes from a maple whirlygig seed, just as a made-up example from my own brain (NOT a real test question as far as I know)--then he'll get that one wrong.
It doesn't mean he doesn't know a million other facts just as important as that one. It just means that he didn't know that one fact. Since social studies and science are quite fact-based, whereas math and reading are more skills-based, it is pretty normal for homeschoolers to score lower on those subjects. My DS is a Viking expert, but he's probably not as aware of the Native Americans as an in-school peer would be because he's just not as into them as the schools around here are. Does that really matter? I don't think so. Not yet anyway. We chose to do different things, but that doesn't mean we'll never cover the topics everyone else covers. So that flexibility of order means that the test is missing his knowledge base on that subject.
...And I'm okay with that. <shrug> No big deal. As long as he passes the test, it doesn't bother me. It did worry me a bit that he scored lower in social studies last year. (Like somewhere in the 80-90% range instead of perfect scores.) Then I talked to an old hand in homeschooling who has GT kids herself, and she said it happens to everyone.
So just know that going in and don't stress...unless you covered the subject matter very carefully and he STILL does less well. Then maybe you should worry...
