If neither OP not their kid, wants to be on the young side at a maths circle, I don't think they would be keen to take maths with the 6th formers, even if it would make sense academically.
OP, try the book "First Steps for Problem Solvers" from the UKMT. It's meant to help students prepare for the Junior Maths Olympiad, which is harder than the IMC and a different style (proof based rather than multiple choice) and which he may find more interesting. Also, a book may feel more complete/meaty than just the past papers he's done so far.
The NIRCH tasks also have a different feel from competition problems.
One of the nice things about the IMC past papers is that the UKMT has "solutions and investigations", where each solution also has an additional question for the student to practice what they learned from reading the solution. If he does IMC past papers, he should definitely do the investigations on all the solutions which aren't obvious/simple to him.
The best books on school maths for mathematically gifted kids are, in my opinion, the ones written by The Art of Problem Solving: https://artofproblemsolving.com/store (you can find placement and place-out tests on each books webpage). I like to use bookfinder.com to find physical used copies, as it scrapes many book selling sites (use the ISBN if you want to search for a specific book, and set the United Kingdom as the destination so shipping/availability is included).
Regarding the UKMT mentoring, he's beyond the Archimedes level, so make sure the teachers nominate him for whatever level is appropriate (you can use the samples on the website for this)