not suggesting you should have known. i wouldn't have had a problem iff dd2 had been 'dunked' in the manner that i 'dunked' her by an lsa if the lsa had been there instead of me. it was every swimming lesson, so every week.
(fwiw dd2 also has cerebral palsy and at the time was non verbal)
the school would surely know your son's medical history? particulary if he has an lsa if not f/t, then at least 1-1 during swimming time? if he was having problems with his ears had you discussed pool time with them? or did you not know that he was having pool time? (i only ask because particualrly hydro-pool time is sometimes on and off)
i just would have thought that if he was having ongoing ear-issues then there might have been some discussion about swimming etc.
do you have medical reports that siggest the ear problems were as a result of going under water, and not as a result of his ongoing ear issues? i wouldn't have thought the febrile convulsions were linked, but i'm not a medic. did he develop a chest infection as a result of inhaling water and that caused him to have the febrile convulsions?
sorry, i'm just guessing as you seem to be sure they are linked?
you asked for opinions whether it was abuse - i'm saying no, i don't think it's abusive. i've seen it done a lot, and all the parents i know are quite happy - some of them were reluctant initially (for the reasons you are upset) but having experienced it, they are all fine and dandy.
i'm not being deliberately obstructive - apols if it comes over that way. i just think the lsa used a jokey term 'dunked' and you have taken it the wrong way.
but it sounds as though you have a lot invested in the argument, so i assume there are other outstanding issues which are not immediately apparent.
but no, i don't think being dipped under the water gently as part of a pool lesson, is abuse, no.