I see a lot of people posting questions lately on different venues about young dogs and new puppies.
In my opinion, way too many people are having concerns about a young dog loosing some interest in retrieving. There are generally two reasons for this happening.
Either you have been making your young dog make too many retrieves in a day, or they are at the age where they are loosing their baby teeth and getting their permanent teeth in. With young puppies, I keep retrieves down to 2-3 for the day. We want to quit with the young dog when they are still eager to retrieve versus pushing them for a few more retrieves and letting them get board with it. When the young dog is loosing their baby teeth, their mouth will become tender and uncomfortable. When this time comes, give them a few days off from retrieving.
I know it’s not the fun part of working with a young retriever, but the obedience is very important. Walk with that puppy on a leash and get them used to walking and even retrieving with it on them. This will make you life much easier when you start with formal obedience. Teach your new puppy sit, here, down and so on. Start making your puppy sit as you put their food down for them and make them sit for a bit before you release them with OK to eat. This is all the beginning of you controlling you puppy instead of them controlling you. Take some time and watch a TV show with the puppy in your lap, so they begin to understand down time and the fact that you are the boss of the pack not them. And to end this, make sure you are giving commands NOT requests.