Need help and advice with soft/sensitive dog

Need some advice and wisdom. Have a 7 month old black lab (male). My plan is for him to be a hunting buddy and pet. Not currently planning on doing field/hunt trials but not out of question. Am very busy at work and I know that is a conflict but am trying to make the best of it. I got him from a great local breeder/trainer with good bloodlines. Not necessarily all titled in his lineage but does have some and both parents are trained and great retrievers. He is a great dog has great natural retrieving instincts and is as sweet of a dog as i have ever seen. Loves retrieving the bumper and brings it back drops it and sits right by my side without me really teaching him that. Have been trying to work mainly on basic obedience. He has a lot of energy and has trouble with stay but we are working on it. He's classic lab and getting into and chewing everything and when i tell him no sternly or have to get on him hard he just cowers and is as soft as I have seen. He's my second lab (had one growing up, she's passed away but was just a pet) and currently also have a boxer. my current lab (bubba) is my first dog that I plan on training as a retriever. Ive gotten Sound beginnings and am starting that. I know I am behind on time but the plan was for the breeder who is also a great trainer to do most of his training but he had some health issues come up and that has been put on hold/delayed. He was with the trainer for about 2-3 weeks and he ran into the same problems as I have with him being very soft and sensitive and said lets hold off and let him mature a little more and try again to as not break him or anything. Im really struggling with what and how to help him because he has to listen and have discipline but I feel like when I get onto him sometimes i am just making it worse. Any of y'all with dogs like him or have trained dogs like him have any advice or wisdom? He will honestly be a phenomenal duck dog I think because of drive and natural instincts but reeling him in and focusing him and training him with his soft/sensitive nature is giving me all kinds of trouble. Any help on directions to move forward or advice would be greatly appreciated as I am admittedly very green. Thanks