My personal take: I don't think there will be much actual value in testing with the SB-LM. What you'll get perhaps is one tester, who after the whole world has gone on to other more modern tests continues to use the SB-LM, giving her personal opinion on how your child stacks up to other children to whom she's given questionable numbers as well, in the context of a sample of children from forty years ago. (I didn't realize it was that old-- good grief!)
I would never, after reading the linked article, pay money for the SB-LM. I might take it if the testing were free, and take the results with a huge grain of salt. I might also consider using other tests, like the SB-5 or retesting with the WISC-IV but a better tester-- and I might want to do this even if I got the SB-LM testing for free.
If after retesting with a modern test your son's PRI or equivalent subtests are still relatively lower, I'd probably shrug and chalk it up to a quirk of his makeup that he's either making more use of verbal reasoning skills than you think, or his visual-spatial skills aren't exposed well by IQ tests.
Aside from identifying learning disabilities or for use in advocacy, I don't think that a few numbers from IQ tests, and the personal feelings of a tester who sees a child for such a brief time, are really all that helpful in making parenting decisions. They are interesting, but not necessary or really all that helpful.
