I used a pencil grip as a child http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=pencil+grip&x=0&y=0 but still hold my pencil "incorrectly"
Offered short pencils and crayons. My oldest resisted for a proper grip for years and I just let it go and it faded on its own. She still has trouble with finger strength, whereas dd2 has no trouble whatsoever.
Short pencils and crayons and some patience on mama's part. That's all it took here.
How old is he? Switching hands and grip is normal in little people and if he isn't getting aching from his grip I wouldn't worry. If it bothers him in the future it might be worth looking at but as the pp said, patience!
That said, I tutored an 8 year old who had a pencil grip that stressed his hand and his whole arm and made him grit his teeth. His teachers wewn't worried but it wasn't good for his stress levels! I talked about a mouse grip with him and it did help quite a bit.
Holding a pencil in a 'good' grip will enable you to move the pen forward and backward just with your fingers, without needing to move your whole hand. That makes a 'squeek squeek' sound and looks like a little mouse moving it's nose (in imaginary mouse hand world!) You can't 'squeek squeek' if your hand is all curved round on itself or you have all your fingers lined up along the length of the pencil so you can use the mouse sound as a reminder about grip without being overly critical.
Great tips everyone! I love the idea of a short pencil -- I'll try it this afternoon! :)