In case you're talking about cash, if you have a job or any source of income, save some cash and give it to him. Be open about it - "You've got a huge bill and helped me a lot in the past, so I wanna help you".

If you're not, help him find a job or means to pay for it.

I'd also speak about being close to help him if he needs with other stuff, but I think you're already doing so.