the first is just very dull, so much grey in the sky and on the ground and i feel like his yellow shirt could pop a lot more.
Also watch your figure-ground relationships it look as though he is stepping on that thing in the background and there is a pole going through his bored.
lighting wise i would try to make the flash higher and add another one on the other side of him. but the shadow the board is casting across his belly is unpleasing.
I shoot alot of bmx and i will get my lights set up and take a couple test shots with the riders standing there, and sometime when they do the trick and arm or sokething will cast a funky shadow and i go to plan B and reset the lights to fix it.
Edit: i kinda combined the two photos into one in my comment. oops
try putting the light in a different spot. the light is hitting the skateboard which makes a shadow(as seen). if the light is behind the camera in this set up, you would illuminate everything you want.
Thanks for the comments. Unfortunately the skatepark is very small and was very busy and the girl I had holding the flash was being impatient haha. The weather was overcast and I did want the sky to come out like that, but agreed the rest of the image is not very vibrant, and yeah clearly the shadow thing. I only have one flash aswell, we started off firing it at his back but It wasn't giving the desired effect, I think my batteries were just running low.