The one I keep hearing about is the TONI - Test of Nonverbal Intelligence. I know there are others, but from what I hear about the TONI is that it is practically pantomimed, like there is NO language what so ever.
If your son takes it let us know how it goes. My daughter has a language impairment, but her private LD Specialist swears she has at least normal intelligence because when you teach her something she learns it and the learning appears to be permanent.
She took the WISC this year and her full scale score was quite low, but the school psychologist said in her report "however this is not representative of her learning style due to her deficits in language." because let's face it, the language portion of the test brings the score way down.