19 is a remarkable age for a dog! You've done a very good job.
I can understand that you really want to help him live as long as he can pain free, but at this age, offering something like methadone (especially without a vet's guidance and monitoring) may just decrease his respiratory system to the point of stopping it.
There's really no other option than having him evaluated first and then try some of the many pain relief meds meant for dogs suffering arthritis and of advanced age.
There are places that may help you and your companion - here are some options:
Also, call your local Humane Society for options and low cost clinics. If you have a Pet Smart or PetCo in the area, they usually have rescue organizations working through them (not a part of them) and they may be able to provide some help.
I respect that you're asking before just trying something - it's intelligent research like this that's obviously lent to his absolutely incredible longevity!