My son is also like that. He is 13.5 mths.
I think now's the age where they test boundaries, especially since we are the ones setting the boundaries.
I also learnt that my son is those who take soft reproach instead of harsh scoldings so that's what I try to do. I will tell him it is ok, that obeying mummy is a good boy and stuff like that.
So far it is working quite well. Though I have to repeat the same thing many many many times before I see any form of improvement.