Yes, you could to give you an idea. Now, to formally substitute a subtest, is not as easy as saying that one would lower the score. A tester is allowed to substitute a score if the tester somehow messed up on it; or if there was some other problem.
For you, you'd be doing it informally, and it would just give you an idea of where he'd be at.
I know that research has been done that correlates math ability to the block design subtest. As I said, since my dd scored low on that one, I am paying attention.
