@Peanutbutterandjams my nephew went there and enjoyed it and did well.
It has a poor reputation, though I'm not entirely sure why. I live in Aylesbury and know some mums who chose Mandeville over PR, which is bizarre to me.
I know nothing of the primary, my nephew went to Little Kimble for infants, then Cuddington for juniors.
I think that the emphasis on primaries is slightly exaggerated. My own DD went to a school in special measures in Aylesbury, but now goes to AHS -without tutoring, so a poor primary doesn't lead to a poor secondary.
I'd have been happy with PR for a secondary if either of my two had failed (I actually looked around it for my DS, but he passed the 11+), as all the secondaries (non-grammars), in Aylesbury are pretty poor.
My DS has autism. It wasn't diagnosed until later in his teens. I didn't think he stood a hope of passing and sounds quite like your DS. I didn't have him tutored either (I'm sadly too poor for that), but he pulled it out of the bag, so your DS may surprise you!
There is also Waddesdon? That has an excellent reputation, but you'd need to live very near to Waddesdon, which isn't near to where you want to go.