Not all dogs go through it, but 6 mos is common. Try to avoid any bad experiences during this period, as that can result in lifelong phobias. Just gentle coaxing, and let him retreat if he needs to. Have fun with your training. Clicker training is great for shaping behaviors like fetch. There's a book I really like, that has a very good explanation of clicker training and ways to motivate a dog that may not take naturally to a task. It's called 'When Pigs Fly" by Jane Killion. You would probably enjoy it.