ok, that statement is different from Cambridge preferring them to do A level maths early.
There's absolutely no advantage that I can see in terms of boredom level though. Surely that would mean he would find maths really boring in Year 12, if it's all content he has seen before, rather than a mix of Maths and FM.
Most Cambridge mathematicians should find maths A level not at all troubling, but the advantage he has now is STEP. He should find he has a lot of spare 'maths' time to really get his teeth into STEP 1 as a matter of routine and then into STEP 2 and 3.
He could, to strengthen his application (though it is by no means necessary, nor a guarantee to help) use this summer to work on Olympiad problems if he is already enjoying challenge problems, scoring highly on BMO1 and perhaps getting into BMO2. UKMT have some excellent books on their website.
I'd hope that he also tried to get some down time this summer too, but I know what it is like with a maths obsessive.