You've received great advice already - the one thing I'd add is that it is most likely worth looking into why there was a dip in the WISC processing speed scores, think through why the school is saying your ds may be challenged with the increased demands in writing in 3rd grade, think through the EF issues you've seen, talk to your ds about what it is he thinks he isn't doing well at, and look for connections.
Did the school do any kind of OT assessment for fine motor skills? Does he have anything in his 504 plan to help with fine motor, or to provide alternative forms of answering questions etc?
It's just a gut feeling but fwiw, the reluctance to write down math problems could be related to the fine motor issues, which could be what's causing the dip in processing speed scores on the WISC, all of which might be tied into why your ds feels he's not good at things, all of which might be pointing to something you need to understand better and address no matter which school he's in (current school or homeschool).
Did the school offer further evals when they "suggested we get an evaluation of his processing", or were they suggesting you approach that further evaluation privately? If they recommended something could you let us know what they recommended?
Best wishes,
polarbear
