IMHO: Velocity at age 10 isn't important.
Learn how to repeat delivery, throw fastballs to location, stay balanced and don't get hurt.
If he's throwing 56 as a 10U, he's fine velocity wise. The rest of it (STAY HEALTHY) is most important.
My son once threw 146 pitches in a LL All Star game as a 10U. The game was delayed over an hour for a monsoon storm, after he took a line drive off his shin, which swelled up mightily while we waited out the rain delay. He went back out and struggled. They brought him out two days later and couldn't understand why he was struggling.
Don't let this happen to you!
Fortunately he's stayed healthy and like others who have posted here, gets tired much quicker now than he did when he was young and either walking or striking out every hitter (sadly, the K's & BB's were about equal at that age).
He just finished his second varsity season as a sophomore and most likely is the #1 pitcher at his 5A-I HS he next two years. Now I go ballistic around 105 pitches...