Fresh or salt water drowning? This will definitely impact prognosis. There's hope for improvement at this time, now is crucial and poor handling will adversely affect outcome. Check for reflex patterns (is he decorticate, decerebrate, can the extensor spasm be overcome, eg with moving head, etc) Need a baseline to work from.
And teaching mum, caregivers, etc what not to do in terms of positioning, handling, etc. I hope you're working with a good OT!
Good luck!