As a PCA for a 10 year with Autism. I know about stemming all too well. What your grandson is doing is tame compared to the 10 year old I take care of. For his stemming, he jumps up and down, claps his hands,and will hit whatever is nearby. One thing you can do is get your grandson a weighted vest, weighted blanket, or a kicking sack.
Also a therapy swing would be a very good idea. Swinging relaxes them and is very calming. Also a small trampoline might bot be a bad idea.
Good luck with this.
Thank you for the info. No, he is not seeing anyone at the moment, but the plan is to get help some form of therapy soon. He did have a behavior and speech therapist last year when he was diagnosed as having a sensory disorder. He's been classified as "high functioning", and we are all in his corner as he develops. He's a remarkable little boy, yet can put you at your wits end at times!
I have to correct myself. In the picture above, he is 2 1/2 years old.