Hey guys. I have read this forum for a long while now and figured this would be a good time to jump in.
I threw a "3 fingered fastball" up until about that age. I was not small (6' 200lb) but had short fingers and felt like I needed the extra finger there. After finally moving to a more typical grip I did add some velocity rather quickly and gained confidence in my control with it rather quickly.
I would suggest having him work with different grips in long toss and bullpens to get him comfortable without having to ditch throw the 3 fingered fastball all together and then slowly fade that pitch out.
Especially with him being left-handed, I feel that moving to a 2 fingered (especially 2 seemed) fastball will pay major dividends in velocity and more so in movement. Also it will allow for a more pronounced speed differential between his fastball and change-up (if he throws one). The benefits are multifaceted.
Just let him determine how quickly you fade the 3 fingered grip out and I think he will see the results/benefits of the new grip(s) sooner than later.