If he is eating and digesting the food normally I think there is hope. While I don't know what the normal life expectancy for a boxer is, I'd guess more than 19 years.
I can't suggest any treatment, but think it best if he is kept dry and warm and not exercised. You didn't say how long the symptoms have existed, if more than a day or two help from a vet is needed, I believe.
Sorry to read this. Could be a number of things including a heart and/or lung issue and he should probably be checked by a Vet. Maybe one would let you make payments or charge it. Wish I could offer more advice.
The average lifespan for a boxer sadly is around 10,we've had ones that lived longer of course. The symptoms have been almost a week I'd say and have fluctuated,we're currently trying natural treatments but without a diagnosis it's difficult.
Sorry for the pain and worry.
The "19" I typed was one of my too many typos.. guess I0+ years is typical of large dogs I understand.
Our research is pointing to heartworm,even tho he'd gotten his shots I believe.
Shots are not going to protect against heartworms. You have to give the monthly preventative medicine. If you haven't been doing that heartworm is a a good probability. Please try to get your dog to a vet. Maybe you can contact local shelters to see if they know any vets that have reduced fees.
If you live in the city travel out to the country. The vets out there tend to charge less.
We live in the Philthadelphia area...everything is more.