I would suggest trying some more extreme angles. I am imagining #4 being shot from a low angle and you positioning yourself facing him. Capture the look on his face. I am not a skateboarder nor I have I shot skateboarders but I am going to assume that to pull something like that off he would need to concentrate. I think that capturing that facial expression would be wonderful. Honestly he looks like he can get pretty high off the ground. Depending on how daring you are have you considered having him ollie over you while you shoot up?
I like #5 but maybe a more interesting shot would be lower with more focus on the artwork on the bottom of his skateboard.
#2 and #3 don't do anything for me. He looks like he is bored and just standing around on his skateboard. For implied motion you would need to have some blur. Slow the shutter down and pan across the scene, following him. This will blur the background but should keep him pretty sharp. Stopping the action in a picture like #4 gives you implied motion because you are stopping a situation which is impossible unless there is movement.
Hopefully this helps a bit. BTW, what part of DC are you shooting in? There is a bunch of nice graffiti on 14th Street heading out towards Silver Spring. Might be an interesting background for a skate themed photoshoot.