I've had a couple of players who get very frustrated and down on themselves when they miss chances. One thing that helped is I searched up a youtube video of Messi missing chances and asked their parents to show them the video. If the GOAT can miss a shot then we can all do it! - then I remind them that "even Messi misses" and it's developed into a bit of an emotional reset for them. 

I've also asked players when subs to watch the game and count something that we've been working on in training (passes / tackles). So they become a mini-analyst for the quarter. So you could ask him to count the shots and goals of other players on your team and the other team to demonstrate the shots to goals ratio. The idea being that they "discover" for themselves that most shots don't score.

Is the player trying to hit the ball too hard? I always worked on the mantra "Accuracy before Power". Once they improve on accuracy they can add the power.

Does the player practice realistic shooting in training? Consider overloaded shooting practice (eg 2 attackers vs 1 defender + GK or 3v2+GK) from realistic distances. 

1 attacker plays in and joins in. The 2 (or 3) combine to get a shot off. There should be enough pressure from the Defender (or 2) to make it realistic but also sufficient room for the 2 (or 3) attackers to create shooting opportunities.

Serve the ball in from different angles and distances rather than everything central. You can add a time limit (depending on age), say, 6/7 seconds to combine and get a shot off if they take too long to set up a shooting chance (to you, to me, to you etc)

Keep reiterating the point you make that not every striker scores every time they shoot. Ask them to reflect on why they have missed - stretching for the ball, kicking under the ball, not making a good contact (sometimes it's just about getting a good surface contact to redirect the ball). If they can self assess, then the question is, so what do you have to do differently to get it on target (you may have to help them with this depending on their age and experience)?

Keep shooting when you have the opportunity - "you won't score from 100% of the shots you don't take"

Shooting and missing is merely a learning experience. Consider the miss but focus on the next chance.

Also, the practice described above, with a change of emphasis can be used for shot stopping for GKs, defending outnumbered for defenders, or quick combination play and shooting from in and around the box.

Hope this helps

Jamie, been there can sympathise. I listened to a podcast  with Michael Owen (I think) talking about his Liverpool days and the sheer number of practice shots in training and the volume of those nowhere near the target. I was staggered. He made the point that it is a percentages game and only one needs count. I took this gem to my U13s who had the same issue as yours, explained it calmly to reassure, they said 'Who is Michael Owen?" Back to the drawing board. Something i do with my current U13s is high volume unopposed shooting, "hear the ball in the back of the net" 5 balls in line across the goal as fast as you can (that gives more of a picture what is happening with standing foot etc. for analysis - video if you can and have them analyse) move the line back a yard at a time. They will miss a lot but such high reps they quickly deal with it and move on

I've found that this happens a lot with kids with ADHD. You may want to check in with the carers to see if there's anything in that. I have also found that this happens in kids who are incentivised by their carers to score - if you score today you get x, if you get a hat trick, you get y, if you finsih the season as top scorer you get...therefore the only value in playing and your only perception of self worth is scoring. If you don't score, you are therefore worthless, a failure.

We've had this with several different players - a couple had ADHD but outside ot that, it stemmed from their carers - grandparents in particular. 

We don't have a a top scorer award at our end of season awards, that has helped a lot.

A while ago, we asked the kids to put their hands up to see how many were on a fiver a goal, how many were incentivised to play their best for the team, win the ball back etc. You can guess the result. We asked the kids to tell thier parents to change their reward from goals, to tackles or tracking back, whatever. That also helped.

Some great advice here Stephen. There are definitely aspects around the below:

Coaching points: Technique related to ball striking. Approach to the ball, surface of foot, non-kicking foot placement, follow through on contact etc.

Practice design: Outnumbered (as mentioned above) but other considerations worth noting; distance to goal in finishing practices and number of repetitions. Most goals are scored within the box (regardless of age) - do your practices mirror this distance? You may look to create a challenge rather than a time limit to finish, use a certain number of balls per round. i.e 10 shots/balls per round. This may become challenging for a player, BUT it may also demand the player to focus more on the above coaching points?

Positive reinforcement: Add your specific coaching points to positive comments. Players will then be more likely to perform your coaching points when being praised specifically for them, especially around finishing. Players will also link your praise to a specific action i.e striking through the ball. 

Hope the above helps and compliments some greats ideas on this thread.

Lloyd