With DD, we already knew she had ADHD. She was diagnosed at age 6 and took the WISC at age 8. So I always figured the slow processing speed was related to the ADHD, but now I am wondering if she's just not getting enough sleep and that is contributing. I forced her out of bed this morning and then had her do some math and she was extremely slow and spacey--major brain fog(even though she was medicated with stimulants for the ADHD). There can be a lot of causes of slow processing speed. I found this article helpful:
http://www.davidsongifted.org/db/Articles_id_10782.aspx

DS's processing speed wasn't great either but the gap wasn't as huge. I think it was around a 30 point gap between processing speed and PRI when he was tested at age 6. Who knows what it would be now 1.5 years later. He also had a traumatic brain injury which probably played a role. His working memory went up when it was retested about 11 months after the brain injury (processing speed wasn't tested again, unfortunately). I think he also may have ADHD but has been diagnosed with developmental coordination disorder. So he is slow with anything involving motor skills.

It annoys me greatly when schools tell parents their kids will never qualify without even evaluating them. If I were you I would put a request in writing, stating what all your specific concerns are. Even if he ultimately only qualifies for a 504, they won't know what to put in the 504 unless some sort of eval is done. If they deny your request for an eval they will have to come up with decent reasons why, and put it in writing, and if it seems ridiculous, you can appeal it. There are so many different special ed categories and if there is anything that impacts a child's ability to learn or function in the classroom, they should at least be able to get a 504. DS's academic achievement is fine according to all the testing he had done, but he still has an IEP (in his case physically impaired). Unfortunately I'm not well-versed in all the laws and technicalities from state to state to be of much help. Check out Wright's Law.