I think something that isn't perhaps being addressed is the harsh reality that in this modern cut-throat, dog eat dog world, there is a real need to achieve via the recognised measures of academic success in order to be able to get through the doors of life. The office boy rarely if ever climbs the greasy pole any more, and those that do, the 'barrow boy made good' are recognised and commented upon, such is the rarity of that 'way up'. Hence exotic's 'world's successful people'. They are the stuff of magazine articles.
30 years ago, boys like my DB, 1 'O' level (but not as daft as that makes him sound: poor SM school!) got on an apprenticeship quite easily at 16. These days you're not suggesting a DC without an English Bacc. would even get a look in! The jobs and traineeships are just not there any more for the less academically able.
So, my point is, I know why a parent might worry about 'average' as measured in school- especially bearing in mind that, frankly, that bar is set pretty low!- even though we know that DCs can bloom later/opportunities to 'catch up' are better than ever they were (though waaaay more expensive than ever!).
For the record, DS1 is what I'd call a 'B' grade student, capable of A when he feels like it. DS2 is very 'average' to the extent I am sending him to private tutoring because I know he can do better and he needs to get an English Bacc to allow him to access a future that pays the bills well enough to allow him a 'nice' standard of living. I know the Eng Bacc was wheeled out as a 'basic measure of secondary educational success' but the fact remains, in my opinion, a DC who gets 2s in KS1 SATS and 4's in KS2 SATS might well have difficulty in passing a GCSE in a MFL.
Many will be howling 'Self-fulfilling!' Your DS2 will always be no more than 'average' because you say he is! My response would be two-fold: No, I know he can do better, hence the gentle but ongoing pressure via tutoring and nightly, self-set 'homework' (largely reading and spelling) and secondly, I feel that our fear of recognising where talent lies -or doesn't- in the UK causes us to pretend all doors are open to all DCs all the time so they do 'one size fits all' Y11 exams- where many will effectively leave school with just about nothing. Far better if that DC's other talents were built on at say 14 so they can access mechanical apprenticeships at 16, for instance!
So my advice to the OP is to see if there are things you can do to help your DC along as if you can, it'll boost their self confidence and ultimately buy them the keys to greater choice in life.
And thank you for your honesty in posting!