Just an idea -
would it help if you gave him a 'mixed bag' of questions - i.e. individual questions of each type and see if he can spot the question type / possible question types/ what he needs to do from the format of the question?
Maybe put them on flash cards & play a game e.g. how many can he spot in one minute?
Normally telling children to read the instructions carefully but guess you need him to be a bit less particular if he's going to get through in the time.
Is he really worried about making mistakes?
Maybe you could let him do it his way but timed so he doesn't get to finish then get him to do another one 'in a rush' & demonstrate its better to finish & get some wrong than not do lots.
With my son there were particular types that slowed him down so he learnt to do the 'quick ones' first which made him feel ahead of time & less stressed before tackling the 'slow' ones. (Obvious other advantage that if he didn't finish he wouldn't have left a set of questions he could have done quickly!)
Hope some of this makes sense
Do hope you find the way that works for your son. This time last year I was really stressed & then over the summer holiday it just 'clicked' & we finally made some progress.