Mine started grammar this week. There is a big difference between grammar schools with catchments and those without 'superselective'
Ours has a catchment. GL assessment in VR, maths and English. They don't publish pass mark but is is rumoured to be 75% ish. If child is 'top table' and has some prep they are likely to pass.
Superselectives take children with highest marks so children need to be scoring very highly to get in - 90% plus.
In our area most seemed to have an hour a week with tutor from part way through yr 5. All 4 that passed from DC school had some tutoring. There are only 24, 50 minute GL test papers from memory to do so we saved those for the summer hols so 3 a week. DD also did a mock for £20 with a local tutor. Sitting 3 papers in 3 hours is a massive thing for a 10 year old, I don't think i had to do that length of exam until my A levels.
For the GL one DD sat it was maths that needed prep as the test is on the yr 6 curriculum and beyond 3 weeks into yr 6. So someone needs to cover it with child before test. Likewise VR there are only so many types of question and you do speed up with practice. There is no way a child can learn how to answer them and answer 80 in 50 mins if they have never seen the type of question before.
At this stage I'd focus on what he feels is tricky and just do some mock papers. We did nothing in week or so before exam.
We used to go through practice test together after and DC often could answer the ones she had got wrong so show him the 2 scores what you got and what you could have got if you had checked answer/not made silly adding up mistake etc. Good for confidence.
Also ensure he is familiar with format of paper - putting a line in computer marked box etc. Make sure he doesn't miss any out. Tell him to mark 'c' for the all if he is running out of time. Basic exam technique but they are 10.
elevenplusexams.co.uk has info on all the types of exam and a forum for each area.
Good luck.