No he wouldn't be because he doesn't have the skills or ability to access the Grade 6,7,8 or 9 questions. Even if he had 4 hours for 3 Grade 6,7,8 or 9 questions he likely wouldn't do well because he doesn't understand the content. So if he wants to scrap a 4, he needs to study and practice Grade 4,5,6 questions and try and get better through hard work and revision not just 4 weeks before the exam but for months. Going over skills he is week in e.g trigonometry. Watching videos, practicing the skills. Then he'll be able to get enough marks on the Grade 4,5 and 6 questions that he should get a 4 or 5.
Also most marks in Maths are for working out so actually if he does attempts many questions he should get quite a few marks. If he's getting a 3, will all the best will in the world, he's not going to be able to do a Grade 8/9 circle theorem question AND even if he had 200% extra time - if he doesn't understand, hasn't bothered to learn, or study for months and do practice questions on circle theorems. He won't be able to do the questions because they'll be way to difficult.
Also teachers and support staff are trained to identify anyone who needs extra time. Also yes I've mentioned slow processing but it won't just be that. They often have to do tests to see whether or not they qualify. JCQ do very regular checks on schools to make sure they are following the rules. Often children will need more than that with examples of dyslexia,ADHD etc. Also a pupil with just anxiety will likely not get extra time they may get a seperate room or smaller room and/or rest breaks. Anxiety on its own won't qualify for extra time.
Finally, it's highly unlikely (stats show) that Johnny will be the one who doesn't get arrangements for exams. Often it's girls because it wasn't till early 2000s that people realised they could have ADHD, dyspraxia etc and they present differently. Even now many girls aren't diagnosed till uni or later because they can mask and training on how to spot in them isn't always done. Then it's actually BAME girls out of all girls who on top of that suffer more, lack of cultural education and again masking.