He can't see the problem though so I can't see CBT being any help until he accepts that his behaviour is a problem. He is in denial.
Absolutely - there's very little you can do about his treatment as it can only be driven by him.
Is he blaming the ADHD when things go wrong, and/or is he expecting that if the medication were working, he wouldn't need to exercise any willpower at all?
He spends money before it's in his pocket
TBH, OP, if he's spending family money recklessly without discussing it or in a way that's detrimental to the family, then neither of you should be wondering if a tweak to the medication will solve it. It won't.
I'm on the right dose of meds for me and I can still think 'fuck it' and decide to eat too much or waste money on frivolous things if I want to. If my decisions affected someone else and I cared about that, I would take that into the equation too. Unless my own desires were more important to me than screwing over a loved one, of course. The meds still let me be an arsehole when I want to be!
ADHD meds aren't going to make him less selfish. And they can't cure addiction - if this is an addiction, he will need help to tackle it when he faces up to it, just like anyone else.
That said, it sounds like he doesn't know how to budget (which makes perfect sense with a lifetime of ADHD, as it's very common to just bury your head in the sand and live in the chaos).
I had to sit down and actually take the time to learn how to budget. He will obviously need to do that too. Medication can't teach skills!
Google YNAB - that's what I use for budgeting- it teaches you to only spend what you actually have and it's great. You could always frame this budgeting issue as a joint problem and something you both need to learn and solve together - then he might not feel he has to defend his fuck-ups by blaming meds or saying 'sorry, it won't happen again' (until it does).