Get him to teach the others. The bigger the gap gets the more limiting "whole class" maths will get for him.
Children taught in mixed ability groups, on a macro (international) level and on a micro level will have better results than those who are taught in differentiated groups. This is supported by enormous international studies (not one of the top performing countries in PISA - flawed I know - groups children by ability or teaches different abilities differently); and there are beginning to be some smaller studies done to support the theory too, for example in http://aer.sagepub.com/content/43/1/137.1.'Accelerating
Mathematics Achievement Using Heterogeneous Grouping'
the performance of initial high achievers did not differ statistically in heterogeneous classes relative to previous homogeneous grouping, and rates of participation in advanced placement calculus and test scores improved.
Agree with mystery - not worth worrying about at all. Teach him some other stuff instead. Doesn't matter if he is ahead in Maths and really doesn't matter if the teacher 'doesn't cater for it' either. There is only so far to go.