GUARDING GROWLING AND NIPPING

My 14 week beautiful male shih tzu appears to love other dogs and lives with 2 cats which he seems to enjoy.I notice the last week or so occasional growling when my 19 year old son picks him up and guarding behaviour displayed growling,barking and snapping when my husband near my dogs bed.I have not had any of this behaviour although my pup follows me at all times and likes to be at least able to see me.I make sure we all check behaviour with a firm no and we stop playing when nipping begins.I worry that this may get out of hand and that I am doing something wrong.I walk my puppy 3 times a day play with him and feed him a vets recommended diet.My puppy greets everyone and displays submissive behaviour lying on back to be petted and wags his tail.I am starting puppy classes this week do you think this is puppy dominant behaviour and what action should we follow.Thanks