Have a meeting with his teacher and try to work out a plan that will be most beneficial to your son. I am assuming slow processing speed was not a qualifier under special education, but it may be something that can be addressed through 504 (I'm not sure though). You might ask for a 504 assessment to be done.

In the meantime, slow processing can actually be handled with very similar accommodations as ADHD. The length of assignments can be reduced (for instance, instead of 20 math problems, he could do 10). Extended time can be allowed so that instead of missing recess, he could finish assignments at home (and if the length has been reduced on work, the amount of homework will probably end up being about what it is right now.) If they haven't done a speech evaluation, that might be a place to start, although they probably did a speech assessment when they did his testing previously. My son received speech services partially because he had trouble with processing what he was hearing.

If he doesn't qualify for special ed or 504, the problem will eventually be that he legally does not qualify for accommodations in the classroom.