I am going to be that guy. What is the driver for September and how many trips will you be able to take with him in the fall?
If you are limited to a single trip in the fall, wait until temperatures decrease so you can hunt your dog for a greater portion of the day. The cooler temps will also help the dog learn faster. Running over birds before the dog can smell them doesn't help them learn.
Also, realize two things:
1: There is a reason people own multiple dogs and rotate them on long trips.
2: The physical conditioning of your dog before the trip will make a huge difference in its lasting power on the trip. That said, your dog sounds young so you do not want to overstress their body this summer in preparation.