If he's a scumbag, 6 years of prison will make him worse. If he's a man, living with what he did is punishment enough. Both ways I find this (and similar) petition like something below a decent man - let the courts and judges do their job. If you want to make the world a better place, start with yourself.
If you think an offender's conscience is sufficient to reform him and, in cases where it is not, prison only makes him worse, then we might as well empty the jails and disband much of the judicial system. I agree that the UK prison system is embarrassingly ineffective at discouraging re-offending when compared to the Scandinavian version, nevertheless, jail is where this person should be.
Considering that he did this when he'd already been banned, left the kid he'd hit in harm's way and drove off, and then thinks that his sentence is unduly harsh... I don't see much evidence that his conscience likely to be bothering him. What seems more probable is that he'll consider any future driving ban with the same contempt as before. At least if he's locked up, he's not on the road.