Potty training can be stressful for kids and parents, so you might as well have a sense of humor about it!
These training briefs ($12), which display a pee guy on the front and a poo guy on the back, will entertain the trainer and the trainee. They are lined with yellow (yes, it seems that was intentional) and should fit one-and-a-half to two-year-old boys.
The characters are actually the same as the Pee and Poo Guy plush toys we saw a while back. Perhaps the set would help your lil guy go from pull ups to undies?