I don't know my oldest rabbit is two and a half so I have no experience with seniors and also with no hay diet. Your diet sounds not right to me but if he survived so long maybe he's adopted this lifestyle, just I think if you'd stop feeding him treats every 2 hours he could start eating his hay. I am sure you know that rabbit's healthy diet is 80% hay and also too much pellets can cause liver problems and there could be stones in his bladder or something else.
Kale contains much calcium and usually limited fruits also not every day food, high on sugars etc.
If you are willing to review and rethink his diet, cut pellets and treats and make him eat more hay I think it can be improved, but again, any changes in his diet must be made slowly otherwise he can stop eating at all and get a blockage, no good at all.
So if you are ready for changes you want to give more information on his pellets, name, ingredients, percentage for protein, calcium and phosphorus, how much daily he gets, what is his weight, if his poos are good and if he's missing any cecotropes etc