Welcome Viv,
If you are comfortable, post his scores from the WISC and any other scores you have. There are many knowledgeable people here who can help analyze the scores.
A speech-language eval should have included both expressive and receptive language evals and scores for both. Expressive language is what comes out (speech and how we use it) and receptive would be, of course, what he takes in (comprehends speech/language) which would correspond to the auditory processing suspicion.
I'm curious how the school diagnosed the probable auditory processing deficit. I have been told that this deficit can only be diagnosed by a specialist in the field. Did they suggest that you seek out (or did they offer to pay for) the correct diagnosis of this deficit? They should if they think they found it. Was his digit span and freedom from distractibility (both on the WISC) low?
Also, if the speech/language pathologist is from the school, most likely they only did a "screening" and not a full language evaluation. If you don't have test SCORES, most likely this is the case which, in my opinion, is not a valid speech/language eval. from personal experience.
We just went through this. Our school did a language "screening" during the full case study, three years ago, and said he "passed." We have had all of the testing redone by outside evaluators. The pediatrician suggested getting a formal eval done at the Children's Hospital and what do you know, "severe delay" in receptive language; he is now 10 years old. All this time we thought language was fine (according to the school).