My crack, expert staff tells me that it's common that a loss of appetite accompanies pain, opiate use, and being 80. And chronic loss of appetite equals weight loss. So, one of two things is the case: either you cannot eat, i.e. you can't keep food down, or you don't want to eat. If the former, you have a situation you must deal with. If the latter you have a problem you can solve by making yourself eat even though you're not hungry. Oxycontin makes some people nauseous enough that it might need to be rethought if you want to eat enough to train effectively.
As for your training, I'd need to know more about the extent and nature of your spinal stenosis before I recommend that you get very rambunctious with barbell squats. You can probably do them if you start light enough and you have some help maintaining good form. Keep rowing and start squatting once a week, and see how you tolerate it. If well, add another workout. Get a rack. I don't see a need to squat more than 2x/week under your circumstances, even if there is no pain or recovery problem. Press as you can, and deadlifts are better than rows for about a dozen reasons. Keep us posted. I'll delete all the gratuitous ass-kissers for you.