I'm not actually in coaching basketball. My only interest is in helping my grandson on how to become a better than average shooter in basketball. I saw in him his passion for basketball.

A fundamental flaw in his shooting (am not sure if you can call that a flaw) is how he shoots the basketball. He uses his both hands in shooting the ball from a distance. I advised him to change his technique by just using his left hand as a support and let his right hand (he's right handed) to push the basketball towards the ring. I told him not to make use of his both hands as they may not always work in unison and therefore may result in inconsistent accuracy. I've observed that he's taking more time to change his shooting technique.

How can I help him?

How old is he and what is his experience level?

For kids that shoot with two hands, they need hundreds and maybe thousands of reps shooting the correct way to break that habit. It takes a while and some decication to make the change. Because he might do it correctly for 10 minutes. Then put him in a game or pressure situation and immediately reverts back to bad habit.

This is partly why we recommend youth shoot close to basket, lower if needed, and use smaller basketball.
https://www.breakthroughbasketball.com/basketballs/size-chart.html

You often need to start away from basket (shoot to wall or spot on the floor) because kids don't like that they miss shots. They need to forget about that at first and just use good technique to develop good muscle memory.

Every day shoot to a wall, then move to basket (stay really close). Take 100-200 perfect form shots a day. In a month, the habit might be broken.

This is also helpful for kids that shoot with two hands.
https://www.breakthroughbasketball.com/store/shopexd.asp?id=132