Best thing for comfort would be to bring your own snorkeling gear, fitted to his mouth and no rent the "one size fits all" on CC.
Maybe they have adjusted gear for this. You might give DCL a call what's possible.
I agree with silly inventor, that the best thing to do is get your child his own snorkel. Dive shops and larger sporting good stores will have a variety you can choose from. It might also be helpful to find a snorkel that has a smaller mouthpiece, as that may make it easier on his teeth. There are snorkels specifically designed for children, you might also look at those designed for women, as sometimes the mouthpieces are smaller.
If your child wears rubber bands on his braces, or uses orthodontic wax, you should avoid both of those while snorkeling. Also rinsing with fresh water as soon as possible after snorkeling will be helpful, as it helps to cut down on the irritation from the salt water.
Ask the orthodontist for suggestions. DS14 plays trumpet and his orthodontist gave him a special piece of plastic to cover his braces when he plays. Your son's orthodontist may have something that will help.
Depending on which snorkel you buy, you can usually buy a mouthpiece separately that forms to your mouth. Basically you put it in warm water, wait until it softens, then put it in your mouth. it will form to your mouth. Then attach it to the snorkel. Usually this means simply sliding it over the bottom of the tube. Most dive shops have them in some variation.