M.C.
Maybe have him sit on the potty seat when he needs to go, and give him a book or something to look at while he sits there. Maybe once he relaxes a bit, and the pressure is off, he will be able to go.
My parents used to have to pour a little bit of water between my sister's legs to help her get going... Not sure if it would work for a little boy, but maybe.
You might also try just running water, or putting a little water in the potty seat so when he goes it makes a different sound. It sounds like maybe he is starting to go, but stopping for some reason. Maybe the sound of his urine hitting the bottom of the seat surprises him.