To me he seems out of ballance. Trying to promote his drive and Im guessing here but You have thrown a lot of marks for the dog and he has learned that its all go for him and by that may have created anxiety for the dog basically stress which has shown its evil face leaving the line. This can come from hunting too soon or for an undertrained dog as well. They just dont know how to handle a situtation that can be very exciting and enticing thus creating a lot of problems you will have to go back and correct especially with birds in the mix. I think its more fair to the dog not to allow it to happen then go back and fix it later.
He sounds like a nice boy so work with him on a level where he can be successful and promote good habits from the beginning and Im sure you will have fine gun dog. They gave you some great advice to correct it but if you take your time and follow a good training program you'll put the odds in your favor against it. Personally I would much rather own a dog with a laid back temperment with soild control. The rest will come in due time. JMO
Best of luck