Maybe ask your Pediatrician... I'm sure they deal with this all the time.
He is at the cusp of pre-puberty... and you may ask your Pediatrician when the hormones in boys start to surge.
Your'e right not to want to shame him... but in school, if the other kids start to notice, he will be "shamed" or teased or worse. Or... as you know, that when males are stimulated, they get an "erection" and other things... and this may pose a problem, visually, for your son... because then others may/will notice.... and how will he hide it?
Maybe continue the stress-ball use at home too...
Have you spoken about it to him?
My Husband has a brother like that... and not to worry you, but for his brother, it became a "habit" and something he did whenever he was stressed and it was his way for "relaxing" and to fall asleep. (My husband and his brother shared the same bedroom growing up, so my Husband "knew" what his brother was doing). Then when he became an older teen to an "adult" he was able to control it more... consciously.
But yes, boys do this (and grown men too! LOL) & it is normal self-discovery... although I don't know to what extent is considered normal or not, or excessive... this is why, you might want to discuss it with your Pediatrician, or at least ask the Doctor how to go about talking about it or curbing it in school. He is getting to the age whereby he will "need" to know about how and when and where doing it is inappropriate or not... and especially in school, where there are boys AND girls... and how others will be affected by it.?
All the best,
Susan