It's time for force breaking at 7 months, but force breaking will not improve your pup's enthusiasm to retrieve. I strongly suggest you get pro help on the force breaking - if done improperly, especially when you describe a pup with lagging interest in the game, you risk souring your puppy altogether. The trick is to keep things fun for a young dog. They have to want the job, else they'll never be more than mediocre at work. If things become arduous for your pup, the best you'll ever get is whatever the pup understands to be the minimum he can get away with. Drive is critical to high performance, and drive is the product of controlled enthusiasm.
It does sound as though your puppy is bored with bumpers, and that's likely due in part to the way you are presenting the game. Think about the reward your pup receives for delivering the bumper to you. What does he get out of bringing it back? What do you do to change up the game and keep it interesting? If he's reluctant on land retrieves but loves the water, let the water retrieves be the reward for an enthusiastic land retrieve. Try throwing a land mark and then upon delivery immediately throw the pumper into the water. Teach him that a quick land retrieve will get him a water retrieve.
There are lots of variations you can create to keep things interesting. Don't always go to the same training area - it can become a drill for the dog. Have you tried tracking games with real birds? Use small frozen birds until force breaking is done so you don't allow bad mouth habits to form. Tie a duck wing onto your bumpers so he gets used to the feel and scent.
