Critical Thinking generally is fab for brighter kids - well, actually for all children. It's all about focussing on higher level thinking skills.
e.g. SYNTHESIS
creating a unique, original product that may be in verbal form or may be a physical object
combination of ideas to form a new whole
what would you predict/infer from ...?
what ideas can you add to ...?
how would you create/design a new ...?
what might happen if you combined ...?
what solutions would you suggest for ...?
EVALUATION
making value decisions about issues
resolving controversies or differences of opinion
development of opinions, judgements or decisions
do you agree that ...?
what do you think about ...?
what is the most important ...?
place the following in order of priority ...
how would you decide about ...?
what criteria would you use to assess ...?
My boys have done some of this sort of thing at primary, and I've done some research on it. I would jump at the chance if my boys had an opportunity to participate in such workshops in secondary.
More able students do benefit if - from time to time - they have opportunities to engage with similarly able students at a higher level. Also the skills he will learn he should be able to use an apply across the curriculum in his studies.
PS If he 'commits' to it and then hates it after 3 months they're not exactly going to stage a public execution are they? What can they do? Is it extra-curric/out of school hours?
HTH