I will be honest and tell you that if a college is only wanting grade 4/5 on entry then you have to know that they are dealing with a wide range of ability in their cohort. My own children went to a place like this but they really push each child to improve their outcome. However mine went in with mostly grade 9s and some 8s.
For things like science you usually need at least a grade 6, a well respected maths teacher on here will tell you that you need a grade 8 to do maths A level and perform well. A 7 at GCSE will typically result in a D or C at A level. A massive drop.
Factually, a low percentage of students do 4 A levels, around 5%, 67% ish do 3 A levels. If he is thinking of uni then it puts him in a more competitive category of candidates. Most students doing a 4th A level do further maths, needed for things like Econ at Warwick, Computer Science at the top unis.
Mocks are not the real thing and he needs to aim as high as he can. GCSE results are a predictor of future A level grades, there are charts tracking students on this sort of thing, it isn't an unknown. The question to ask is if a child comes in on a grade 5 what A level grade do they typically get? Not every child is heading to uni. Plus what do you expect them to say in assembly? The grade curve is applied so some students no matter how hard they try will get a grade 3 or 2 because they don't perform as well as their cohort.
Uni entry grades are the lowest they will take an applicant in on. If he is looking at an oversubscribed course at a top uni and their entry grade is A star AA then just know they statistically they take the applicants predicted 3 A stars or more, then the A star A star A and then the entry grade applicants. Again, this information can be found to show the grades of those on the course.
Also this isn't a level playing field either, he is competing in his GCSE against kids at Eton, Westminster, St Pauls etc all the children whose parents have paid a lot of money to see their child do well. The grade curve applies to all of them.