Hi, I am new to this, but have a problem with my 11 year ol GSD. He has recently been put on Metacam and is doing great, can run up the stairs now

But the problem is .... he is rufusing to go out for walks, he hasnt been out for a week now. I can get him to the end of my drive then he turns about and heads back to the house. What can I do, and why is he behaving like this. Thanx