Hi DavidNDC,
I actually work at a dive center where SNUBA is incorporated and I actually find it extremely easy for younger divers to get involved with diving at an earlier age through this program. It's actually really cool because the youths that we take out on SNUBA dives typically come back and get certified when they become of age, which appears to be the goal of the folks over at SNUBA International. I've brought it to the attention of the president of SNUBA my concerns over previously talked about lung over expansion and there has never been one case ever reported which is pretty astonishing for a company that's been around for 30 years. Probably for the fact that your not even at one atmosphere it's relatively safe. I fully recommend getting your little guy involved with SNUBA and get him comfortable with breathing compressed air under water so he can get his certs done when he's of age! Hope this helped. Have a great time in Mexico!
Regards,