Obviously there's a considerable history here and, without being in possession of all of the facts, it's not going to be possible for any reader to give a properly informed opinion.
If I'm reading you correctly, you say that your ds had smeared excrement in the 1st cubicle. If that is the case, it would seem probable that your ds needed to be divested of all of his clothing but IMO it was not appropriate for him to be stripped naked in a corridor when, presumably, other rooms were available to clean him.
If you are saying that the 1st cubicle could not be used because of smeared excrement, with the implication being that your ds had played no part in its condition, it still would have been appropriate to remove him to a private room to clean him as trying to perform this task in the close confines of a toilet cubicle would not be particularly easy.
It would seem that scant regard was paid to the dignity of your ds, but it is a moot point as to whether this incident constituted abuse per se as it needs to be considered in the light of additional information from the relevant staff.
If your ds was left alone in a soiled condition for any great length of time it would seem that neglect may have occurred or, at the very least, staff may have failed in their duty of care. However, again, additional information is needed before any opinion can be formed.
Was the hearing you referred to called by the LEA to address a (your?) formal complaint? If so, you will have a right to appeal any decision and, depending on the outcome, can refer the matter to the Secretary of State for Education or apply for Judicial Review.
I know how exceedingly difficult it can be to take on bureaucracy in an attempt to hold individuals to account for their actions it can be a nightmare worthy of Kafka. The best advice I can give you is to endeavour to detach yourself from the minutiae as much as possible and play them at their own game continually put the ball in their court.
All I can say by way of encouragement is that dogged persistence has been known to produce results, but this often takes a considerable toll on the time and mental health and physical well-being of the complainant which is why so many give up.
Have you sought legal advice and did you have legal representation at the hearing?