Let me preface this with the fact that I am not an advocacy expert in autism. However, procedurally, I can give you some pointers.
First, push hard for the PWN. You are absolutely entitled to this. Look up and refer to your state's law/regulation on this and/or your state's department of education guidelines and compliance standards and refer to them in your request IN WRITING (email is fine). If this doesn't work, file a Child Complaint with your department of education citing failure to comply with required PWN procedures.
Second, request an Independent Educational Evaluation (IEE). Assuming your state is similar to others on this issue, you do NOT have to provide them with a reason for this request, but the reason would be your disagreement with the decision that the sensory issues set forth in the school's evaluation do not affect him in the academic setting. The IEE will be at "public expense," so you do not have to pay for it. Hopefully, the IEE will come back with a direct correlation of his sensory issues to academic impact and will make recommendations for services and/or accommodations. If accommodations only, you may have to go the 504 Plan route for now until he is older and there is more proof of the academic impact.