It really depends a lot on how much the child enjoys the competitions. If it's a straight choice between following a more advanced course and competition prep, and if he doesn't actively dislike the latter, then I'd say the competition prep is more valuable hands down. But there might be a third option that might be even better, and especially might be so if your DS doesn't like the competition per so: starting with some material (book, paper, whatever) about some aspect of maths that interests him, even if it isn't course-shaped, and having the freedom to explore, define and attempt to solve his own problems, go off in whatever direction is interesting, etc.
More is true than that many top mathematicians have no great record or interest in competitions: in fact the vast majority don't have such a record/interest. Competition maths is great but it's weird in several ways. It presents you with problems to which you know a solution exists, in fact even a solution that can reasonably be found in X minutes and using techniques within set Y; and it does not give you any input into what the right question is (other than the freedom to e.g. ask a more general question in order to solve the set one). In this way it's profoundly different from "real" maths, though undoubtedly the skills it develops can be useful. OTOH maths as presented in maths courses is also profoundly different from real maths, so you take your choice!
