Has your son seen you pee on the ground? I did the first time with my son just so he would see it as a normal thing. Only think is he thought it was too normal and would prefer to pee on the side of the house instead of coming in when playing.

Funny story two weeks ago we were hiking back to the car and I took him by a bush so he could pee. when he pulled his pants down I noticed his underwear had a wet spot. I asked if he had wet himself and he replied "no last time I peed on the tree and it bounced back." I told him from now on pee next to the tree