Judges said the boy, who is now 19, and also suffers from epilepsy, had his human rights breached after he was subjected to unlawful disability discrimination and false imprisonment.
He was awarded £28,250 in damages following the incident at a swimming pool in Acton, west London, in 2008.
However, the Met has signalled its intention to appeal against the decision.
The boy was physically removed from the swimming pool and forcibly restrained after he jumped into the water fully clothed.
The court heard it was the first time police in London had been found to have subjected a member of the public to inhuman or degrading treatment, and to disability discrimination.
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A police spokesman said they were giving the findings of the hearing "full and careful consideration".
He added: "We will be seeking legal advice and take forward any learning as appropriate.
"We are making an application for leave to appeal today."