Training an old dog new tricks

My dog is a 4 yr Dougue cross Lab and has the stubborn streak of the Douge breed. I would really like to teach him to fetch so that I can an excercise him in the afternoons but he just won’t do it. He was very good at fetch in his first two years but then stopped doing it. Any advice would be greatly appreciated as he gets bored and digs holes everywhere.

Regards,

Monty81