I am no expert.
I am on my 2 nd dog.
6 months is still young.
Stop the water retreivers and work on whistle stop and come in tweet until it is 100 %. On land.
Make him stop and sit every time. No retreivers just walk blast ,stop sit. Over and over.
5 minutes couple time a day.
Put a check cord or line on him. Let him snoop around blow the stop. Make him stop and sit every time.
Don't be harsh just firm.
Once he has stop down. Than blow you come in tweets. Make him do it. A little treat every third time will give him the idea.
Blow come in when you feed him.
When he is 100 % on land. Do easy short water retreivers. Use the check cord.
You can live with almost anything else, but you have to have obedience or you got nothing.
You have to keep revisiting the rules. To keep him on task.
This is what I would do. I am not a trainer. This has worked for me.
