This is a great question, I think there is some test of memory. I could not help the same as Dottie, but if you post the subtest scores that might be good...
I think the speech delay could affect his scores in all the subtests except I think there is a hands-on portion here and there, like a few puzzles; was the tester prepared beforehand with the knowledge that your child has a speech delay, and are they experienced in working with such a child?
The work you mention he is capable of is pretty amazing, but I am not sure the wppsi looks at computation at all -- no questions on 'what is zero', 'what is 2+2', and definitely NOTHING about what is the cube root of anything.
