You're thinking about this all wrong here. You buy a mask like this (obligatory Amazon link) and wear whatever earbuds you want.
Does some kind of mask over his mouth work? Like a hood or maybe even just a mask with stronger strapping so it doesnt slip? I'm not a fabrics person but there may be fabrics that offer some ability to breath but limit the amount of water that gets through (probably cant stop the water getting through/the mask getting soaked, but if you can reduce it substantially to only a very small amount, that might work). maybe? or something like this. Or this ski mask (maybe cut so the nose is clear)
a dam or mouth guard etc only works if its gripped in the mouth. In which case, its basically the same as keeping his mouth closed? If it had a one way valve (I dont know of anything like this but assume it can be made) then the valve would have to prevent anything entering (from outside to his mouth), which means it couldnt be used to breath in (but could be used to breath out).