It is good that his mother and you are investigating the alternatives and exploring the pros and cons of habit appliance. Let me direct you to an article regarding habit appliances such as rakes, cribs, and gates. Here is the link: orofacialmyology.com/wp-content/uploads/2019/11/2013-april-orofacial-myology-news.pdf
Quite often therapy cannot be effective with a Class III underbite. I’m going to ask you to visualize the jaw/mandible as it literally “carries” the tongue with it. The tongue is attached to the floor of the mouth so if the jaw is forward, then the tongue has to be forward along with it. Now if the tongue and jaw are forward, one would have to curl the tongue awkwardly backwards in order to try to use the “spot” where others naturally rest their tongues. But for someone with a Class III, especially a substantial one, it is totally unnatural to rest on the spot and thus cannot be habituated to the spot. All of this can make the tongue appear to be too long, but it is very unlikely that it is so. Hopefully, this will help you see the connections of the Class III, the appearance of a long tongue, and the frequent negative results associated with habit appliances.