On Monday this past week, Briggs told me he needed to use the bathroom 2-3 times with enough time to make it there. So I just started taking him on a regular basis. Every time I took him, he went. He stayed mostly clean and dry the whole day. On Tuesday I got him some big boy underpants. On Wednesday we busted them out. Briggs thought his Thomas undies were great. Since then, he's been using the bathroom regularly when we take him and has had very few accidents. He's not initiating much on his own and doesn't seem at all bothered when he does have an accident, but it's definitely been a successful (and pain free) start to the process.
And he sure does look cute in his undies.