That's very frustrating isn't it? You are right to be concerned and to look at fixing this before it gets worse. One thing I thought to ask right away - is your boy neutered? Sometimes hormones can make a dominate dog worse and he is at the age where dogs sometimes think they can challenge the "pack leader". If he isn't neutered I would suggest you check in with your vet about getting this done as well as making some changes in your home.
#1 if you are able enroll him in an obedience class and do either # 2 or #3.
#2 work with an animal behavorist or
#3 Teach him that you are the "pack leader" and do the Nothing In Life Is Free (NILIF) program.
You can google this program and find sites that tell exactly how to implement the program but basically it means acting like a good "pack leader" so he will respect you. (Growling is a show of disrespect or trying to take over leadership). Some things you do is making your dog work for everything - food, attention, exercise, etc. I.E. he has to sit before you put his supper bowl down. Make sure you eat before feeding him (pack leaders always eat first and lower ranking animals eat after). Always make sure you are the one who allows him outside and you go out before him, he has to follow you. For the time being don't allow him to sleep with you or be higher than you on the couch. Allowing him to precede you out the door, be higher than you on the couch, sleeping in your bed, eating when you eat are all signs that he is equal to you or higher than you in the pack and just enforce his right to be disrespectful or to let him think he can take over.
A dog that knows his place in the pack is much happier and feels more secure. After all, it's your job as pack leader to provide food and keep the pack safe and he won't have to worry about it.
If you have a spouse or children make sure they participate in the program as well. The dog should always be below them in pack ranking also, otherwise he will try to push the kids/spouse around too.
Good luck with your boy, you can PM me if you have any questions about this program. It works wonders with dogs.
Teresa