Unfortunately, without the additional information requested in my first/previous response, I will not be able to give you detailed advice to your question, but I will be able to give you a general answer, which I hope will be of assistance. If you are able to supply the additional information later, please do so and I will then come back to you with a more detailed answer.
You do not say what the disability is the your son has been diagnosed with.
The school would have a legal duty not to discriminate, and should make where possible reasonable adjustments for your son. However, sometimes it may not just be possible to make some changes. That would not be discrimination, but unfortunately just a reality.
They should not be making harsh comments on his absence if it is unavoidable because of medical reasons.
As for grades, and marks on work. Normally extra time and other allowances would and should be made for students with difficulties, but obviously that does not mean lowering marking grades so as to give a student a higher mark because of any difficulties or disabilities they may have. That in itself would be discriminatory to do so.
If you feel you are being ignored, then I would suggest writing or calling the head of school and asking to arrange a face to face meeting to address your concerns.