Covering Up (not sure this is the right place to post but...)

So I live at home with my sister and her 2&1/2 year old son. He think's he owns the place and walks into rooms like its nobody's busy. I love him to pieces and never really had a problem with this. It has resulted in him seeing me naked a few times which I never thought was a big deal. 
The other day he walked in on me in the bathroom naked and asked me where my penis was. When do we need to start worrying about privacy?