I empathise OP - and I agree with pp that really it has to get to a crisis before things happen. I apologise for the length of this, but there are so many similarities.
We had this with my uncle (unmarried and no children) who lived over an hour's drive away. Not quite the same as with you as, with me not being his child he wasn't expecting me to do stuff, but the neighbours were concerned he wasn't managing, his house was in a state, his garden - well put it this way, I didn't realise he had a shed in the back. Milk bottles with sour milk piled up, bins not put out but overflowing all the same. Obviously struggling to walk. My mother (also in her mid 80s) would phone and he would tell her all was fine, he had been doing the garden and tidying up - all lies - utter lies! Put us off visiting saying he would be out until one day we just rocked up and we were shocked. Piled up rubbish in the kitchen, dead flies, broken fridge/freezer full of rotting food - at this point I'm in my 60s, and whilst up for some tidying, wiping and hoovering, this was beyond me, never mind my poor Mum. We did what we could (including emptying that fucking fridge/freezer - I was heaving) and came home and started trying to get him help.
Upshot was we tried to involve social services, told him to accept help - he would agree to our face and then refuse all help, wouldn't let them in the door. We spent hours on the phone and we may as well have pissed into the wind. The only good thing was the doctor - I wrote to his GP expressing my concerns whilst understanding things were confidential, and the GP did visit and do what he could (uncle did let him in - I think he thought it was a social call).
One neighbour was very nice and genuinely supportive, but the other (a police officer too, who I thought might have understood the situation better) phoned me and rather snottily told me that something had to be done as he couldn't go on like this. I asked her to suggest something I could do that I hadn't already done more than once - she tried a few suggestions, I told her that I'd already done those and why they hadn't worked, and she rang off moaning. The bottom line is that he had capacity and was allowed to refuse - even if it was to his detriment - and refuse he did. He refused all help, lied to all the agencies who tried and lied to us. In despair I asked the GP what usually happened in these situations, and he said in his experience a crisis usually arose - maybe a fall or a broken bone - and things usually changed from there.
In fact a few months later the neighbour couldn't get an answer from him and phoned me. I phoned the police, they did a welfare check and he was found dead in front of his TV. He was 89 so a good age, but I still think had he engaged with those people trying to help him, he would have had a good few years more.
Interestingly, when I phoned the police officer who attended the scene, he said how shocked he was at the state of the house (and a little judgmental I think too). He said they'd had to wear masks because of the fridge/freezer which we'd emptied. Apparently uncle had just carried on using it like a cupboard. I told him we had moved heaven and earth to try and get him help, but that he had steadfastly lied and refused all help and SS had been happy to leave it at that.
Do as pp have said, be clear and consistent with him and any agencies (hospital, social services etc) and ideally in front of both parties, that you cannot help due to your own health, the distance and family commitments. If he says he's fine, point out the ways he so obviously isn't. There isn't any more you can do.
But I feel for you, it's a shite situation.