How old is he? Does it seem to be aggressive growling or playful growling?
Maybe he's bored or needs more exercise? Have you tried wearing him out a little?
My Muffin growled at first when I'd come to pick her up, but I think she was scared. I just said No in a firm voice and then held her so she'd know she was safe with me. She never growls now.
If he growls when you pick up his toys then I'd say "No" in a firm voice and continue to pick up his toys in a matter of fact way, while not looking at him.
If he's chewing on his leash then take it away and hand him some puppy chew toys or bully sticks. Puppies are going to want to chew stuff their whole lives. So by giving him acceptable chew things, he knows what's ok or not.
Sorry, I hope this helps. Maybe others have better ideas
Good luck
