You need to go to elevenplusexams.co.uk there are a number of forums there, look for your county and there is likely to be a discussion about the school he hopes to go to. There may also be some threads already about marks to aim for on each practice paper. Without knowing the school and the papers he is doing, and having some experience of that test we can't really help, but you are most likely to be able to access that specific help on the forum linked to above.
Around here a super selective only takes the top 5-10%, there may only be a few in each county and the entrance requirements are often ranked so for example the top 100 boys get a place. A selective is more likely to have a definite pass mark and the nearest 100 boys scoring over X will get a place.
Around here there are both selective and super selective schools. To just pass often they only need to get half the marks on the actual exam, whereas to do well they will need to get higher marks.
It is important to go through the paper afterwards and spot where there are gaps. It may not necessarily be because they aren't good at maths, but because they haven't been taught it. My ds gets lower scores in maths than the others, although in class he is one of the top, simply because he hasn't covered all the 11 plus work yet (in yr4). In the English and verbal reasoning he can score well on simply because he is very keen on reading, and non-verbal reasoning is to an extent not easily taught and more 'intelligence' although there are tricks that you can learn. Maths if you haven't been taught ratios or algebra then however good you are at your times tables and co-ordinates it is a big ask to expect a 9/ 10yr child to be able to figure it out in an exam situation.
There are loads of you tube videos which explain techniques, but he will need some support in terms of thinking about which questions he routinely gets wrong and how to tackle them. If he has a tutor then this is what you are paying them for, if not then (and I know it is hard with a baby) you need to go through each answer and spot common themes. Is it always the same areas of maths or does he get confused with worded questions and though he knows his times tables has difficulties when it is loaves of bread in an oven or plumbing call out charges.
I really would go to www.elevenplusexams.co.uk/forum/11plus/ this should take you to the forums page (although there are lots of other useful resources elsewhere on the site). Look for your region and there is likely to be info on the school you are interested in. Plus you can sign up and ask questions.