DS pretty much consistently gets 95% on school maths exams. Never 100%.
I didn't think the PMC bonus round questions were excessively difficult, with the higher levels (JMO, IMO), the start getting less trivial.
Here's the paper he did:
www.m-a.org.uk/resources/PMC_Feb_2011_Finals.pdf
He got q12 wrong, putting 4, whereas the answer comes out fairly simple if you draw it on a piece of paper, it's a sort of Maltese cross.
He also got q15 wrong, putting 19 I think missing the two 7s required for 77. A bit of a classic brainteaser that one.
He also got qs 21, 22, and 24 wrong. q21 is easy if you write it out as algebra. q22 possibly a bit wordy/confusing? He said he didn't understand q24, so I guess too wordy again.
I had him do the JMC 2012 paper as practice (he's too young for it at the moment), he got 79, which is a silver, which is obviously good for his age. He made a couple of silly mistakes on the numerical questions, i.e.,
"What is the smallest four-digit positive integer which has four different digits?"
A 1032 B 2012 C 1021 D 1234 E 1023
"Which of the following has exactly one factor other than 1 and itself?"
A 6 B 8 C 13 D 19 E 25
and he got pretty much all the shape/geometry type questions wrong.
I was going to ask his maths teacher if he could do the JMC, but I didn't realise that he had already done the PMC and not got through to the Bonus Round, so it perhaps seems less of a reasonable request. However I would say that the PMC seems to be a lot more wordy, and I'm sure that is confusing him, as he does have Aspergers.