Laying down is a conditioned response to the pressure involved in the process of ff. ff is a fluid process after the dog comprehends what is being commanded to do. Move the dog forward to the object he is being commanded to fetch. I will be willing to bet this dog has a very good sit response and may actually be fairly steady already. Me, I would use a pinch and warm him up with walking fetch for a week and go into heavy walking fetch after he has been conditioned with the pinch. When he starts to lay down move forward and apply pressure with the pinch.