Ailing aged kitty

This summer I have been caring for my brother's ailing 17 year old cat, Dupree. My brother is out of town on a job so I have been taking care of him since he's had a few medical issues in my brother's absence. He's not been eating much and has gotten very thin and he yowls mournfully a lot. I think that he's not feeling very well but I also think that he misses his person, that being my brother. He's always been a sort of one person cat and that person has been gone all summer, so part of his problem might well be psychological in origin. I've been taking him to the vet a lot to have him checked for a variety of ailments and I've had to give him medicine a lot as a result. I feel really bad pinning him down, forcing his little mouth open and giving him medication, but he's actually pretty good about tolerating that. He's also surprisingly good about vet visits. The first time I had to take him, my biggest concern was whether he would ping-pong all over the car and make it difficult to drive. That is what happened when he was a very young cat, but now that he is much older, he's very good about sitting in the passenger seat and letting out the occasional yowl in protest. At the vet's office, he very dutifully sits in my lap and allows me to pet him and talk to him. A few short years ago, he would never have allowed me to so much as pick him up, let alone hold him on my lap, but he is becoming rather sedate in his old age. Now he sits in the car while I drive him to and from the vet and protests on the way there but is usually very good while coming home. I think he knows that it's all over and that he'll be back in the comfort of his home soon. Tonight he was a tad cranky even after I brought him home and gave him medication prescribed by the vet, but he eventually calmed down once I petted him and talked to him and reassured him that everything was OK. He just wants to be comforted by people, that's all. As long as he is being talked to, petted and given some affection, he seems to be OK. I think he is just very lonely right now. But he doesn't seem to feel well on top of being lonely and I hope that he returns to some semblance of health soon, despite his advanced age for a cat. I take him back to the vet on Thursday for an injection so we'll see what progress we make in the coming days. I just hope he regains some appetite and starts eating again. He's gotten alarmingly thin, in my opinion. But his meow is strong and he does go in and outside, so there's obviously a will to keep plugging on. Maybe he's just waiting for his person's return later in the summer. I can only hope so.........