they need to do when his mind is getting distracted in game.

You can practice teaching your hands to function without thinking about it in two ways:

1) through similar distraction (and practicing focus)

2) through boredom.

Catching hundreds of passes whilst bored and tired after practice has a very similar effect in training your unfocused/distracted mind to still make catches.

He should practice as much as he can in stressful situations and with distraction, but it’s much easier to get 100+ bored catches per day after practice.
Lots of great receivers do this (eg. St. Brown, Rice, etc.)