We had this same problem w/ our younger. She didn't want to do it on the potty so she stayed in diapers. Kept them dry, just did #2 in it... lol. Anyway, she did it ONCE for my husband while I was at work. This is going to sound SO gross and it's such a guy thing, but my husband took a PICTURE of it and her next to the potty "to show Mommy when she came home." Yeah, thanks. I was SO disgusted, but to my complete dismay... IT WORKED! That was when she started and kept doing it consistantly! Ugh, I HATE to give him credit for something so gross, but it's kinda funny. She was SO proud! (You should have seen her cheesin' for the camera! And he named the photo "Megan's pride"... he's sick =) So, maybe... MAYBE he'll do it for someone other than you. Just a thought. Hang in there... it'll happen soon enough. Here's another thought... will he be going to preschool? (Not immediately, but in the near future?) Maybe telling him he has to do that and be a really big boy to go to preschool?
Hope this helps!
If not, at least it probably made you smile =)
~K.