Start taking him at set times to keep up his confidence until he is willing to tell you he needs to go again.
Potty training usually is a case of two steps forward, one step back. It is common for kids to do great and then stop. Usually it's because they are stressed over something. It could be a new school, a new teacher, a kid pushed him at the playground, or even a developmental leap in a completely different area.
at 4:24 PM on Jan. 19, 2009