Hello!

There is some evidence that doing exercises that require movements similar to the stereotyped movement can reduce the stereotypy for some time after exercising. For example, if the stereotypy is flapping, maybe you could test if it works for your child doing some physical activity that requires hand/arms movements, like ball tapping/throwing, every day, for at least 20 minutes. In a study (I will leave the reference at the end of the message), the researchers tested the effects of ball tapping/throwing for children's hand flapping and rocking. The result is that ball tapping/throwing reduced the flapping but not the rocking.

Reference: Tse, C. A., Pang, C. L., & Lee, P. H. (2018). Choosing an Appropriate Physical Exercise to Reduce Stereotypic Behavior in Children with Autism Spectrum Disorders: A Non-randomized Crossover Study.

so many reasons can cause a stim. Have you looked into biomedical research? I

I gave justin a squeeze ball it's like a stress ball, and every time I seen justin flapping his hands I would ask him I'd he wanted to play a game weather it's a board game or give him a plastic bat with a plastic ball take him in the front yard to play. When you see your child flapping his hands keep your child busy with something that they like to do. Like games, art. It worked for me. I think they do it when they are nervous