I'm not sure about the law currently or the CMS but if you're not married, it's I imagine it's unlikely they can claim against your income.
I'm sorry your partner is unwell. My best wishes for his recovery.
Some questions/thoughts on your OP:
"I collect him (at least an 8 hour return journey) And drop him off"
Do you think he should be delivered? Could he travel by rail/bus? Would you prefer not to see him?
"we buy birthday and Christmas presents Etc and clothes when he is with us"
Do birthday and Christmas gifts count as maintenance? Do you pay something towards the clothes he wears the rest of the time?
"his ex is now saying she is going to csa for Maintenance"
Her timing seems cruel, but is there another particular reason she's chosen to pursue this now? Has she never had financial support from your DP during child's life?
"I asked what she wanted/needed"
Perhaps she needs money, OP.
"when his child visits hardly any clothes Are packed"
Do his clothes always go home with him? Has his mother had financial support to buy clothes? Maybe she is reluctant to part with expensive essentials that she bought in case they don't come back in the state they left?
"yet his mother has all the latest gadgets, clothes and regularly goes out partying and drinking"
Does she earn? Does her family have money? Your DP is half responsible for the child. Do you think he should only contribute financially if his ex is so poor she's unable to clothe herself or socialise? Is the child neglected by her in order that she can spend on herself?
"mindful that any monthly allowance wouldn’t necessarily go on his child"
So he hasn't paid anything before? Does his ex get to decide from afar what he spends his money on?
I understand it's a very difficult time for you and DP, but it sounds like he hasn't been a great parent all along. Sorry.