My FIL, another bloody lawyer was exactly like that last year - he was clearly unwell and did all he could to put anybody off. I told him that if he didn't do something about his symptoms and I found out, I'd hunt him down.
When the daft bugger was admitted to hospital two months later with (what turned out to be at least the second, if not third) a stroke, I walked in with DP and my first words to him were 'told you I'd hunt you down, didn't I, you silly sod?' gave him a hug and sat down.
He then needed bullying to engage with the physios and to accept treatment. Not nastily, a 'nope, you need this, don't try and argue with me, we'll come back this evening and check you've done it' and rather than sitting down to be the voice of Doom or being all passive and helpless about it, telling him that if he wanted to argue with me, he'd bloody well do it from a sitting position, and if he didn't want the pneumatic boot things on anymore, he was going to have to carry on with treatment so he could walk around and prove he didn't need them anymore. Because I needed him back to argue/debate with me about anything and everything again ('who else am I going to argue with if you're not around?').
He could overrule his daughter and his sister, as he'd always done. He tried it with DP, but he told FIL, smiling 'not this time, Dad. If you refuse treatment, I'm setting Mitz on you and you're on your own there, because I'm not daft enough to try and argue with her'.
It wasn't unpleasant or my really throwing my weight around, it was a case of a stubborn sod who was used to getting his own way all the time being met with somebody who was just as bloody minded and, as I told him, quite happy for him to hate my guts, but he'd have to do the speech therapy in order to tell me to bugger off himself before I'd listen.
He recovered reasonably enough. Because he was lucky, the treatment they offered was fantastic and whilst he is only 66 and moves like a 90 year old now, he does actually move - and isn't dead.
Would being uncharacteristically matter of fact about his situation be enough of a change for him to think 'actually, I'd better do something about this sooner, if only to prove her wrong/prove that I'm right'?