I taught him to speak first, sometimes he would just show his teeth and snip at me when I gave the speak command. When he did that I said "Teeth!" and gave him a treat. He caught on and now "Teeth" means to flash his teeth at me like that.. it's called 'catching a behavior'. It was very simple because he did it on his own, all I had to do was mark it with a word.
Thank you - I was going to make him wear shorts, but he really didn't like the idea of not being able to do his business lol so I sacrificed one of my shirts.