Check his release time by videotaping it and playing it back in frame-by-frame. At his age, he should get below .8 seconds on a pitch down the middle. His longer term goal should be below .7.
As for arm strength/velocity - it is important, but it is easier to shave tenths off of release time than on velocity, as CADad said.
With a .9 release, if he throws 70 MPH initial velocity, that would be a 2.26 pop time. Increasing velocity to 75 MPH will shave exactly 1/10th of a second.
Obviously it is easier to improve release time from .9 to .8 than to increase velocity by 5 MPH!
I did the math once and made up a spreadsheet. Going from 70-75 MPH shaves .1 seconds. Going from 75 to 80MPH shaves .08 seconds. Going from 80 to 85 MPH (very few HS catchers throw this hard) shaves .7 seconds, and going to 90 MPH (dream on!) shaves .6 seconds.
So, the "returns" to arm strength are diminishing.
A strong arm is very mportant in a catcher, but if he is popping a 2.4, the real issue isn't arm strength, it is release time.
Get it on video to see exactly where he is, and if it is above .75, which I would virtually guarantee it is, work on that first.