C.W.
Bravo, that he was trained before 3. I believe boys are easier than girl, especially if they are taught to sit on the toliet facing the tank.
They do however regress for many reasons. My older one was done at 22 months and I never thought my little one would get it. Finally at 26, months we had it, then lost it.
He was afraid of the dark bathroom and going by himself. Once he knew that he could wake Mommy up, and got the super big hugs for staying dry, and 2 months of the routine, it was done and over with. We restricted drinks after 7pm, and he went 3 times before bed at 8:30. "Come on ..., it's time to try. If nothing comes out, nothing comes out." This told him it wasn't up for discussion, but it was okay if he really coulnd't go.
Hang in there. He'll get it again.