Good call on waiting till 18 months Matt. I agree with this practice if you feel he isnt coming out enough. Each dog is very different.
You know that the dog will bark agressively, chase a man, and at least lunge at an approaching stranger. Giving him time to build confidence and maturity in his day to day life might just be the ticket.
My current male never saw a decoy until he was 16 months and he turned out just fine and I've seen dogs as old as 3 years old do great on thier first bite.
I agree, he already serves some purpose as when he is left out in the fenced in yard everyday while we are at work he will meet any stranger at the gate and bark. Its enof where the delivery guy gives him enof respect that he now delivers the package a good 50 feet in front of the driveway gates. I highly doubt he would actually bite anyone that decided to jump the fence.