I doubt the dentist would reccomend it and most coaches won't allow it. its not just the metal on his teeth its the tension being pulled across them.

I said this in another thread but if you hold a peice of paper with one hand and hit it it probably won't brake. If you pull the paper tight and hit it. It tears.

And the end of the day though see what his dentist says and if a coach would allow it. it may be that he only has a pernimant brace for a few months then all he has to wear is a retainer which he can remove when he boxes.