Where we live there is no mainland-style NHS. We pay for GP appointments (roughly £43 for a routine visit, children usually half price, and those who are in receipt of some kind of benefit pay less or nothing, depending) although hospital referrals are free (although nothing to stop you paying to go private at that point to jump the waiting lists).
Dentists are more or less the same - all 'high street' dentists are private, but (unlike the average GP fee per visit) prices can vary considerably. MIne's about £40 for a check up. Hygienist is more at £65 per visit (but I have also had brace work, and now have metal retainers behind my teeth which need a really good clean to protect the teeth)
I haven't had any work doing for... well, can't remember the last time (and I have a mouth full of large fillings that do need replacing now and again!). I pay as I go - they invoice me, I pay it, and I only pay for what I need at that time, be that a check up, the hygienist or a replacement filling.
But I do go every 6 months. I know many who only go once a year, but I have stuck to the 'old way'. And it pays off - for me - because when I went last Christmas, my dentist mentioned an 'possible' issue that 'might' become a problem, but I could take measures to avoid it. So I have been. And when I went back in July, the 'possible' had disappeared. But if I'd missed that Christmas check up, and only stuck to my summer ones, I might have had an issue by now that couldn't be reversed/resolved so easily. So, in that respect, and for someone 'my age', I think we do need to look after our teeth more than the youngsters who have grown up with, generally, better dental care than we had a couple of generations ago when we were children.
My DP stubbornly refuses to give up his Denplan (he goes to a different dentist to me and the DC) but he's on the top monthly rate, yet doesn't get the benefit. It's all "But if I needed a crown..." which, admittedly, are £400-£600 a pop. But he's already spent more than that on "But if..."!
So, while I have no choice but to pay anyway, the upshot is that - in the OP's DP's position - it's better value to 'PAYG' rather than have a monthly subscription, especially if you're not actually getting anywhere near the value of it (or getting any value at all, in the OP's DP's case!). He could always put a little extra into a separate account to top up at the time if the bill was more than a standard check up etc. But, when that separate account has reached a reasonable 'buffer amount', it can just sit there. No need to add more to it until it's been dipped into.
Sorry, that was longer than I intended.
TLDR: Op, cancel his plan until he notices, then tell him to PAYG after that! (And tell him mum to mind her own!).