Any number we could give you would be arbitrary. Every kid can be different on what their body and arm can handle. Therefore, I would key in on how your son feels after these practices. If his arm feels fine after these practices, I would not be too worried about it. If his arm does not feel good, I would have him cut back on his throwing for the next few practices.
I would actually be more concerned about the number of throws he is making rather than the length of any one throw. After a certain number of throws, each pitcher will hit a wall where the next pitch is liking throwing three pitches. At his age, this level is very low at something like 70 to 80 pitches.
Hope this helps,
Baseball Training Tips