This thread about Max is important! I'm sorry to hear that he is in pain and has been hiding out alot.
I don't know alot about IVDD but I am familiar with arthritis and Metacam. My RB dog was on it from the age of 14-16 and it helped her so much with getting around better and being less painful.
My cat Paddy was prescribed Metacam last week because his joints have been under too much strain with his weight and he may have arthritis. The vet said to give him 5 drops for 5 days and then wean him down to 1-2 drops a day. It is remarkable how much it has helped him and I hope it will be the same for Max. I have been mixing it in with wet food and he doesn't seem to notice it. Within an hour, he is walking with so much more ease, it is very noticable. It might take a day or two for it to start working for Max? I think Medi-Cal makes a dry food that has glucosamine in it. Hills makes a food for joint care called J/D too.
I wonder if accupuncture would be helpful for his condition.
Prayers going out for you and Max.