Male shih tzu 20 weeks old behaviour changing ... suggestions?

My puppy is really good with most things: house trained at 13 weeks and good with other dogs and people. He enjoyed early puppy socialisation and training classes at which he excels.

But on occasions he will growl and has snapped at my 17 year old son ... I feel this may be guarding behaviours but he only displays this with my sons and occasionally with my husband. We tell him no and walk away and ignore him for a while but I am not sure this is the best action. He never displays this with me as I am his main carer.

I am going to make appointment to have my puppy done asap as he also is humping dogs and people.

I wonder if this will help and appreciate any suggestions.