Work on your composition. In each of these, the kid is right dead centre...it really gives them a 'snap shot' feel, rather than a great portrait feel.
The flash is used effectively for fill light, and ti matched the ambient fairly well...but it's not dramatic or exciting...it's just flat light from on-camera flash.
We don't know the little guy...to us, he's just a kid with a dirty face, runny nose, dirty clothes, messy hair...and a blue ball. To you, I'm sure they impart a lot more personality. It's a tricky task to be able to have other people see someone's personality in a photograph. It's especially hard when the model is someone that you know so well (or is your own child).