to ask DS's bio father to buy his uniform?

[Aldrin]

I need to say right of the bat that DS's father does give me money (privately not through CSA), broadly in line with what he would be asked to pay if I did go to the CSA.

Times are really, really tight for us (me, DP, 3DCs - younger two are mine and DPs). DP's hours were cut and we're both looking for extra work but it's just not happening.

Got DS1 to try on his uniform today and it's all much too small - too tight and too short, especially the trousers.

Texted his father to ask that he pick up a couple of pairs of school trousers this weekend (his w/e with DS), and maybe another pair of shoes.

He said no, that was what the money he gives me is for.

I don't disagree but it's gone on food and rent and bills - all just as necessary for DS.

AIBU to think DS's father is being unfair?

I agree with your ex as well, he pays you regular maintenance and also has the expense of feeding etc when he has his son. You get the maintenance, child benefit, possible tax credits for your son so essential clothes should come out of that.

It was your choice to expand your family with a new partner, if it was at the financial cost of your first child then that was a decision you made not your ex.

As he's paying child support then yabvu!