I trained my kids with m&ms, i would take them to the bathroom every 15 minutes, even thou they didn't have to go, and every time they do number 1 i would give them an m&m , both of them called me when they had to use number 2 because they had gotten used of me taking them to the bathroom. Every time they did number 2 i would give them 2 m&ms....
It work with both my kids... boy and girl...

I did start that way with No 1 so I guess I can try and reinforce with No. 2

Have you tried to go to the bathroom with him or use a toy to demonstrate, I was told those helped people a whole lot. My parents had it where if he went to the bathroom, number 2, to show them and he would get a reward whether that was a sticker, toy, or even a treat it helped encourage him to want the reward. He still does it at age 6, but none the less he goes in the toilet