Peeing on trees

So, this is my three year old and this is his favorite thing to do right now... pee on trees! He asks me to do it and I say no but he will sneak it in there. We could be standing on our front porch and he would rather run across the yard to one of the big pine trees just to pee on it! His daddy would be so proud!!! He does it all over the place, its like he is marking his territory, making sure all the other kids know that he has been there! Today, our little neighbor friend (she is about a year older than him) walked over with her hands on her hip shaking her head and told him that we always pee inside!!! It was perfect!!! I was hoping maybe if he thought she didn't approve that he would be embarrassed enough to maybe use a toilet but that probably wont happen, but I guess we will just have to wait and see!!! And if you are thinking to yourself why I dont stop him and make him go inside... I try!!! But he is so quick by the time I realize what he is doing, he is mid stream and what am I supposed to do???