So sorry if this advice is too much or unwarranted, but here is my suggestion:
I fully believe in the PBIS system. It's this idea of focusing on the behavior not the child.
When I read your comment I see that it's not actually his reading that's the problem, it's his behavior about his reading that is. (Considering he is on level or approaching on level and smart) So, as a teacher, how would I try to improve that behavior? Improving his reading skill (while great, don't get me wrong, and I'll give suggestions on that) will only teach him anytime he's up to a challenge he'll need to be extremely frustrated until he masters it.
So, I would do this: Make a chart. Call it something fun for him, maybe with his name that rhymes with something peaceful, calm etc. I could call my son's Corben's Calming Chart or something. Depending on if he likes to, you could even have him draw/color around it to personalize it. Everyday have him read something. Not something that is too beyond, but something you know he would normally get frustrated with. Everyday that he's able to read the passage without getting upset give him a sticker. Note: I wouldn't say he has to get X amount right. That's not the behavior you're trying to fix. You can correct him when he's wrong, but keep it positive. Like, "That word is pronounced X, it's ok, just breathe, read it again, and move on." And honestly, just by reading everyday his skill will improve anyway. Set up a prize that he gets after so many stickers. Maybe even a grand prize if he can eventually go X days in a row (I wouldn't reveal the "big" prize, though, until you're sure he can reach it.)
Bonus points if you can get the teacher on this to some degree. Like maybe a sticker sent home each time he wasn't frustrated during reading time so he gets double stickers for the day.
The idea is to teach him the skill about what to do when he feels frustrated because this will not be the only thing that'll frustrate him in life. I would give him a certain technique to practice for when he does (ie. pause, breathe in, breathe out, continue).
As to actually giving him a boost to his reading skills I recommend some of this:
This book was amazing for my daughter:
https://www.amazon.com/Teach-Your-Ch.../dp/0671631985
I used to do it with her once every few days and supplemented it with Bob books:
https://www.amazon.com/Bob-Books-Set...ords=bob+books really helped with fluency.
We also live near our library so take the kids there once a week to pick out books. I let them pick out w/e they want because I want them to feel the freedom that they can choose what they want to read vs. what I want them to read. I don't care if the book is "too childish" or "too beyond" and would never tell them. This is about helping them develop a personal reading interest. If it's too childish I can have them try and read it, if it's too beyond, I read it to them. Honestly, as a HS teacher I think this is huge. I have found that girls are much much more likely to read books than boys. And I think this is because schools tend to teach a lot of books that are more interesting to girls, and downplay those that might be more interesting to boys. So anything you can do to help him find what he likes to read, and let him keep at it no matter the subject, is a great thing imo.
Thank you to you and everyone else who said so. I know I am not the best there is. I bet you there are many more who could teach these kids more in a year than I could. But I know that kids need this kind of teacher just as much as those that are skilled at getting kids to absorb a huge amount of information, but lack the ability to create those bonds. I'm glad to be the type of teacher I, am though. And I know I'm improving every year, too.