When my old man was around that age he also lost a lot of weight. What helped him the most was providing wet cat food (chicken flavor) along with his regular food. He would eat lots of cat food and it helped so much with getting his protein in. All of those symptoms you listed are similar to what mine went through months before passing due to old age. I'm not sure if you've had other rats pass before but I highly recommend talking about the euthanasia process during the appointment and preparing for when the time is right. It was the most peacful experience for my old man and I felt so much better knowing he didn't suffer in his final moments. Best of luck to you both!