Clearly something very seriously wrong. No three year old can carry on like that for 8 hours unless in serious pain or terrified or drugged.
A simple temper tantrum would have exhausted him and he would have fallen asleep.
Its pretty clear to me the child has some kind of autism or developmental delay. The non-stop energy, and non-variant, non-verbal screeching is a key sign, particularly for a child of his age/ size. Also, his total inability to be confined is a signal.
Perhaps mom simply didn’t know what to expect when she took him on the flight