How old is his Mum ? She will be doing this for at least 10 years, or he will need to find a replacement woman.
Realistically he wont want this “palaver” forever.
As pp said, let him fail, let him flounder, he should learn what you had to carry, & wrapping him in cotton wool just creates a false image to the DC,
Being flexible is for serious child based reasons, not because he wants to go to a barber/tattoo aptmt/football match/other
If for example, he wants to swop days to go on a stag 5 day bender, he needs to reciprocate if you want to go on holiday, /other…or pay for child minding if his mother won’t.
He wanted this, He actively chose it.
If he finds it hard… “oh dear” “TWAT”