Why would a 3 year old boy start urinating everywhere after being potty trained for a year?

Anything new in yours and his life, like routines or a pregnancy etc? Is there anything physical wrong with him like a UTI for instance? You need to go over and see if anything has changed since small children is very impressionable. It can be his way of protesting against something new or if he is insecure about something. Let the diaper back for a few days and then prepare him for that "on Monday it's gone" so he knows.