Timing of his shutdown was certainly coincidental with permanent teeth coming in and it is highly common for a dog with tender gums to be less than completely enthusiastic about retrieving during this period.
I am a huge advocate for force fetch training regarding if a dog is perceived to be a "natural retriever" or not. Force fetch training and solid obedience are the cornerstone foundation of all more advanced retriever training. If you are not upntonthe job, pay a pro to do it for you and it will be money well spent.
My retriever was well on it's way to being force fetched trained 3 days into it, with very little "pressure" needed to accomplish the desired response. Regular reinforcement for about another week after starting FF and it was on to a firm foundation for other retriever related training.
Good luck with your retriever.