C.C.
Hi S.,
I know you've already gotten a lot of advice but I thought I'd throw mine out there too :)
I was told by my pediatrician to just let him do it on his own. He won't get an amazing cleaning but it will get him into the habit and eventually he will learn how to do it correctly - and he did. Plus, its a lot less traumatizing...and he won't hate brushing his teeth :)
and another thing that helped was getting him a spin brush - my dentist let me know that it was the absolute best thing to do since most kids aren't as thorough as they should be. hope that helps! Good Luck!