I'm so sorry for your pain and suffering. Our family had lost two of our Jack russells within the last couple of years. One we had to make a heart wrenching decision to put her down, as it was a long trip to the university that could treat her and whether she survive or not it was $2-3K, and if she survived she may have to be treated for diabetes (which the vet think it was) for the rest of her life. My mother can't keep a schedule like that and we didn't have the luxury of time to really decide on options (she was about near comatose, harshly breathing and seem to barely be hanging on). In the end we let her go. At least the one good thing was when they had prep'd her for travel, she was already under meds and had a catheter in her so the process was more smoother without further discomfort for her.
Our older jack russell had died at home earlier this year. On the day before she died, we considered taking her to the vet for either last options or to put her to sleep. She wasn't walking but she seemed fairly alert, so we hesitated and figured to wait for the next day to see how she does. Wished we could've given her something more for the pain. But she had 3 surgeries (two within the last year) for tumors, so they think it was internal tumors (the vet believed it was tumors on the brain and her kidneys, because of her symptoms and her bloodwork).
I also hate to advise this but you might should let him go. You can choose to keep him home and try to just make things comfortable as possible for him till the end with pain meds, etc. or take him to the vet for a quick end. Might ask the vet if they do house calls, see if they'll do it at your home where he'll feel most comfortable. Chemo is hard on a full grown human, let alone an itty bitty dog, especially a senior one. If the surgery/treatment was guaranteed or have a very good success rate, then I'd say go for it, but those are usually 50/50 at best to much less when the dog is older and has had other health issues. We hesitated with our older JR girl, Chloe, and she suffered in her last several hours for it
I would rather they go peacefully than watch them suffer. I know it's not an easy choice, but there are at least a couple options you can do for his end. If you're wanting to do the first option, consider talking to the vet about some good pain meds or anything that could keep him at ease as much as possible. Let him eat anything he wants, etc. (sans anything considered toxic, of course) it can be hamburgers, human food and so on, again, within reason. I know that may sound like a bad idea, but at this point I'd give him anything that would make him happy.