You may wish to send an email or letter to numerous people (this sort of thing has to be in writing). State that as per section xxx of federal law, you requested an evaluation and that it was refused, which is a violation of the same law. Note chapter and verse the way Wrightslaw does, and ask why they're failing to comply with the law.
I totally understand what you are saying. I feel like if I start referencing laws, it's time to hire an attorney. I'm out of my league.
Nagging feeling: if I have to pay a lawyer to keep my kid in a school, why even. KWIM?
Yes, it's amazing how a Free, Appropriate Public Education can start to run you into real money if your child has special needs.
I do think it may be time for a letter from the special ed attorney specifying what the law states. They are saying his disability does not have any effect on him, educationally speaking (this is what they mean by "no disability" in their letter). We know this not to be true. Sometimes a lawyer will write the scare letter for you pro bono and only make you pay if it doesn't work and you need ongoing legal services.
Your other option (before the lawyer) is to meet with the superintendent of schools, or whoever is directly above the person who sent you this letter. Have this rejection letter and the NP report in hand, and show them. Let them know you believe that this decision is both incorrect and is a violation of the law. Ask them to do the right thing. May work, may not; cheaper than the lawyer route and lets the people in charge know what's going on under their noses. Gives them a chance to change course and do the right thing. Of course, they may have directly ordered the bad behavior.
A call to the State Department of Education can sometimes make things move ahead, as well. I have found this strategy quite effective, and sometimes remarkably speedy.
Apparently the school folks are hoping you and DS will just evaporate into thin air. Since he's attached to his program and you're both operating under such stress about the probation, I wouldn't let this go. I'd call them on it.
Unless change is ordered from above, they may well fight you every step: the educational evaluation comes out "no problem" (at which point you and lawyer or advocate again call their attention to the NP report); they grudgingly admit the ASD may be something after all but they insist he needs no help or services of any kind. IME this kind of resistance typically melts only if you are willing to be very persistent.
Personally, I have found it worthwhile to persist. YMMV.
