Aqua Gaurd is fantastic I use it all the time- even though it comes sticky to stick on your skin I always put tape around the edges to just make sure it is fully sealed. But it is so nice because it gives you the freedom to shower and I can image for the beach too. I would still be really careful so he doesn't get it wet, but this will help alot!
Hopefully going to the beach with this on will help he feel better- and feel like he has no limitations
Good luck