Have x-rays been done? Bloodwork? The first thing you need is an accurate diagnosis.
At age 13 (senior) I don’t know how aggressive you want to get about treatment, but I would consider making an appointment with a Veterinary Internal Medicine Specialist.
I would direct your questions you have to the vet, I would not give him anything (over the counter meds, supplements) unless recommended by a vet that has examined him.
Call the vet in the morning and leave a message for him to call you back when he has a minute.
