
How to help a chronic bad passer
Check their eyes. They might actually have a vision issue.
This might come across as stating the obvious, but are they of below average intelligence? Sometimes kids just need one or two things to think about, any more and their brain get fried. If they are really smart, then sometimes they go the other way and over think a situation. KISS.
Perhaps, start of with a simple rule of only pass if you have a clear path and you have eye contact. Maybe a second rule that he has to call the receiver's name before he passes.

You can't expect good decisions from someone who has to watch their dribble.
If he's been playing for three years and he's a guard that has had to dribble the ball, he can probably already dribble without looking at the ball, he just doesn't think he can. (Like most kids can touch type these days, but a lot of them still look at the keyboard out of habit)
Concentrate on getting him to keep the ball in his hand longer. Not batting at the ball but not palming/carrying it either. This is easier to demonstrate than write about, I hope you know what I'm getting at.
Its something he can readily fix on his own time, and any dead time at practice he should be dribbling a ball without looking at it (Waiting in a lay-up line, when you're knocked out in bump, whatever). Tell him to do it for just 5 minutes a day. On the way to school, whilst watching TV, whatever. If he does it, he'll be dribbling without watching the ball inside a month and will be a far, far better decision-maker in general. Heck 1 minute a day for each hand will make a difference.

PS
you referee with a whistle and call travels, fouls etc , a defensive foul is an additional point to the team in possession , an offensive travel is a loss of a point etc. If when playing even numbers a defensive player "looses" their player , call a halt ask them who they have , if they cannot take only one step and physically touch them , then their team has an immediate loss and gets pushups - this teaches the offence to also try to lose their player BEFORE looking for the ball. - also the players CANNOT call out for the ball , it is all done by visual decisions - forcing the passing player to have vision.
this is designed for u10 and u12 mostly , but should do OK for you u14s.

1) get a 1 or 2kg medicine ball and have the kid pass it to an adult at home at varying distances and varying styles ( one handed - two hand chest , two handed overhead etc) this will sharpen and strengthen his physical ability for the mechanics of the pass.
2) run a competitive passing drill at training. say 2 teams of 5 in the half court . Player with the ball can only bounce the ball twice before passing, each completed pass is one point, you cannot pass back to the player who passed to you. the team must make 8 points/passes in a row to win. If you fail to make 3 passes in a row you immediately lose and the entire losing team must do 8 pushups. Only use the half or quarter court.
you can vary this drill by changing the numbers on each team. Less on the defence makes it a defensive drill of anticipation and defensive faking. Less on the offensive team makes it a read and react offensive drill with pass faking highly important.
cheers

Does he dribble with his head up? Or does he still watch his dribble?
Most of the worst passers I've played with/seen are players who make snap passes on less than half a second of actually looking around. Or to put it even more simply, you never develop court vision if you don't look at the court.
That's an easy fix if he's dedicated to improving like you say he is.
If that's not the problem, you probably need some more rigid rules (that the rest of the team is aware of) when he has the ball. He will still miss some opportunities, but until he can regularly hit a known/expected target, don't give him free reign to "create".

One thing I always got taught was that in many instances, it is the player receiving the pass or attempting to receive the pass who is also at fault.
I think its important to reiterate the idea of teamwork and put the idea into the players heads that you have to help the passer making the pass.
Making a "V" cut, "swinging the wings", teaching your players to seal their defender off and teaching players to call for the pass with their outside hand. Those methods will help players get more open, which should reduce the risk of the pass.

Video it and review it with him. That way he sees where he is going wrong. Then work on strategies to improve. Sometimes they have to see themselves do it.

What position does he play?
One thing you can do is teach him the hand off and make sure the other players are aware, that if he does not have a very good option he is to hand off the ball.
Also what type of bad passes ...missing open targets (eg point to wing) or bad options (eg feeding low post from above the free throw line).
If it is the first - just work and work on basics chest pass and hand position. Your job as a coach is to improve him (ie you may not cure him).
If it is bad application of team rules - sub him each time he screws it up, but don't sit him. I will often sub someone but I don't sit him - I may ask what he did wrong or explain a teaching point but then I send him straight back in on the next sub.
Because I do this, the boys understand that this is only going to cost a minute or two off their playing time, the boy I subbed in knows its a quick hit (in fact I'll say I just want to talk to Fred so work as hard as you can for 2 mins).
