chris,sundridge do a flotation suit in sizes between xxxsmall to kingsize.its called the sundridge sas2,if you buy one,you get a flotation vest the same size free of charge.as for lifejackets,most yacht\boat chandlers sell them.
there are usually ads in boat fisherman mag.
Chris,
I used to work refurbishing ships, whilst working in Genoa the coastguard came aboard and tested the lifejackets on the liner we were refitting, an 8kg (eight) weight was attached to a jacket and thrown in the harbour. It sank! So did subsequent jackets.
Don't buy secondhand lifejackets! (Or sell the kid.)
I don't plan on falling in mate but knowing my lad he's probably going to manage it at some point. Suppose I could just feed him polystyrene for a few days before he comes out.