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.