Potty Training

Okay, I would consider me and Adam to be pretty good parents, but his whole potty training has been the toughest challenge!
"They" always say it's harder training boys, but man this has really been difficult. Dallas is just getting to the point where he's waking up dry and always saying when he has to go pee pee/minus random accidents. Meaning, I feel comfortable going to the store with him in big boy underwear. Well, the other has been quite the challenge.
I actually made a poster board and bought really cool lightening McQueen stickers so each time he does #1 or #2 he gets stickers. He has about 15 #1 sticks, but only 1 #2 sticker. I've told him that if he get's 5 stickers for #2, I'll take him to Toys-r-us and get him a toy, but this could get very expensive.
Any suggestions or ideas to help us or motivate him would be GREATLY appreciated!