What (if anything) can we do?

[NotaDisneyMum]

DSS-7 lives with his mum and stays with us from Thurs to Monday every other week, so hecatrives and leaves I'n the school uniform provided by his mum.
She and my SO have disagreed I'n the past over the size/condition of the clothes- it got to the point where SO provides a separate uniform/shoes that DSS wears only on the Fridays that he's here.

The school shoes hecwears when he's with his mum are a size too small, and obviously cramping his feet. SO has asked BM whether she would reinburse him if he bought DSS some new ones this weekend, but she has refused - she says that she has asked DSS and he says his shoes are fine.

What can we do? Poor kid is not comfortable for 9 out of every school days - but BM won't accept that and refuses to put him in shoes that fit!

She told SO that if shoes he bought went into her house, shed throw them away!

Well, I bought DSS a new pair of shoes to wear to school tomorrow - I don't know what will happen to them once he gets home to his mum, though I hope she lets him wear them - he was so chuffed with them, and immediately asked SO whether he could show his mum

We took his "old" shoes with us for the fitter to check; he very tactfully said "yes, he definitely needs a new pair, probably a whole size bigger" - but didn't quite hide the look of horror on his face at exactly how cramped DSS feet were

and throw away the old ones. That's what we do/did. DSSs would turn up in school trousers too tight/small/short and I just threw them away and bought new ones in asda for a few quid. End of.

Last time SO put clothes he considered to be too small for DSS in the charity bag - his ex screamed abuse at him in front of the kids on the driveway - accusing him of theft.

He's put the old shoes in DSS handover bag - it's up to her what she does with them

He's 7, coming up 8 (going into yr 4 in Sept)

His mum helps out at the school, so leaving shoes there for him isn't a guarantee, either, unfortunately

I agree that asking for reimbursement is inflammatory - so SO and I have decided that the best thing to do from now on is for me to buy extra bits his kids need that their mum isn't providing.

That will hopefully stop a situation where his ex leaves the children to go without because she knows he'll buy what they need leaving her with the CSA money to pay for family luxuries rather than the essentials. She might also be less likely to chuck away something I've bought - but we'll see about that.

I'm at a loss really - I just don't get why she would allow DSS to suffer just to prove a point with SO?