Put a dummy in his mouth and walk with him on leash; checking him all the time. If he tries to roll the dummy or move it in his mouth, say "no" while tapping him under the jaw. When he gets to the point that you're comfortable, start moving away from him. As your comfort level increases, move further and further away until he's at the point he will retreive without chewing the dummy.

My 2 cents.

Good luck