Behavioral changes can be bought on by many things...
Without any more details it is very hard to have an idea of what he's really doing and where it could be coming from.
For instance are these "rude" behaviors things he could have learned from his environment? Does he attend child care or creche or playgroup or hang out with older cousins?
Is it possible he is experimenting to get reactions?
He may simply be maturing and coming to the realisation that he has thoughts and ideas of his own and may be starting to attempt to give himself a sense of power of choice by reacting to things he doesn't like.
As I said it is hard without many details....