Thank you for the replies. In regards to the question 'does he have any problems throwing the ball right now?' the answer is yes, to a degree. Sometimes he has a clean release, other times he appears to be grabbing the ball, other times it appears to fall off of his hand. I think that he has too much reverse (as BrunsRich has stated. And yes, a youth cannot always tell you what is 'wrong' with the grip.)

BrunsRich, would you recommend to start at 0 reverse and 0 lateral (as solid9 suggests), or 0 reverse and leave the lateral at 1/8 away from palm? I could install a 1 1/8 thumb slug and start at 0.

And again, thank all of you for your help!

I thought that I would provide an update to this thread.

I reduced the youth bowler's span from 1/4 reverse to just under 1/8 reverse. It ended up being right between 0 and 1/8 reverse. I also remeasured the thumb lateral pitch and grip span using recommendations from JohnP (thanks!).The lateral was reduced slightly from 1/8 to closer to 0.  I re-slugged his equipment and incorporated the changes.

It took a few practice sessions to let him get used to the changes, but he now releases the ball much cleaner. Yesterday in JV bowling for his high school his scores were 194, 200, 176 for 570 - high on the team and needless to say much improved in all areas.

Thanks to BrunsRich, Bowling300900, and JohnP for all of the very good advice!

I am curious as to why you had reverse, much less 1/4 reverse, in such a short span.