Question

I have a neutered male golden retriever who is 12 years old. He has had arthritis in his back legs for a couple of years and is fed on Hills JD, has synoquin tablets daily and has a course of 4 Cartrophen injections every 3-4 months. His last cartrophen was mid September. I have noticed he has become a bit lethargic and seems to have lost his appetite. He is still eating but just not with his usual enthusiasm. I might have to get down on the floor and pretend to eat his dinner to make him interested so he will eat it. I have noticed he will eat soft food more readily but still with not much enthusiasm. This is out of character as he normally bolts his food down. He also seems more tired and just not as interested in things going on. I took him to the vet on Saturday wondering if he had toothache and the vet gave his mouth a good check and said his teeth are in great shape and told me to come back in a week or so if he doesnt improve. I have now booked an appointment for Monday with his regular vet and wanted advice on what questions I need to ask and if there is anything I should note or observe before the appointment. Also are there any tests I should make sure the vet does for him?