Potty Training Woes

Ok so he KNOWS when he has to go now. He KNOWS. He begged me for a diaper and I said no he goes on the potty so he ran to his room. I ran after him with the potty and he was just starting to pee on the floor so I put the potty under him and started cheering and saying YAY and he ran away and continued to pee on the floor.  Ugh.

He loves to 'practice' pee on the potty. He has no issues sitting on it, standing in front of it and aiming and saying "Peeeeee" but as soon as he actually has to go he begs for a diaper or runs to his room to pee on the floor.  

I don't know how to work with this. This is frustrating.