Next time, don't lay him down. This puts an already fearful dog into a more fearful submissive position. The best thing to do is redirect and walk the opposite direction. Once the growl starts, the dog is already in a heightened state. So no need to give him any more opportunity to keep him in it or to have it rise. Removing him allows him to learn to trust you that you will control any situation and he won't have to revert to his own devices.
When you walk away, after a minute or two, and no further reactions from him, praise him for calming down and "shaking it off". You will get much better results and see a lesser reaction time to recovery at calm.
LISA ~ Lily LaRue - 4/07 boxer; Buster McDoogle - adptd 6/07 Vizsla/Besenji mix; Annabella Kanicki - 5/08-7/09 staffy; Mr. Smiley - 4/10 Boxer/pibble mix; Fozzy Bear - 5/10 Red Heeler/pibble mix