Well, if you are scuba-diving, then better to learn Marcante-Odaglia than Frenzel. Frenzel requires to remove the regulator from your mouth and close your lips, and possibly also make use of your cheeks for getting more pressure. It is excellent for free diving, not so practical for scuba diving...I use Valsava, but am trying to learn Frenzel.
The OP here is referring to a pressurized chamber, where his son is probably breathing with his nose, as he is not submerged in water, nor using a scuba system. In this case, Frenzel is the top method, allowing to use pressure created both in the mouth and in the nose, for maximum effect.
But when scuba diving, wanting to keep the regulator in your mouth, you can only use the air trapped above the soft palate, without making use of your mouth. And this becomes the Marcante-Odaglia method, not as effective as Frenzel in creating very high pressure inside your nose, but much more practical while breathing from a scuba system.
Then most people, including many diving instructors, do not really understand the slight difference between the two methods, and talk about Frenzel also when referring to open-mouth equalization.