I think you could use "Marine Goop", a silicon-based glue that is
used on boats and underwater purposes like sealing pool
lights(available at hardware stores). If he dives deeper than 30'
your tube may have problems with pressure. Have you thought of an "O"
ring set-up? Some hardware stores carry "O" rings of different sizes.
I'd try your local dive shop too. I'm curious what your solution will
be. Hope this helps!