My family had your exact problem and we ended up creating our own company selling fidgets to help others. 🙂
I would start with how do you see him exerting his energy most? Is it whole body bouncing, leg bouncing, fidgety fingers (repetitive motion), squeezing things (resistance), or maybe chewing on clothes, pencils, etc. There’s so many ways to fidget, but you can usually narrow it down to the particular sensory input needed such as strong tactile input (rough / spiky), soft tactile (fuzzy / smooth), resistance (pushing / pulling / squeezing), oral (chewing / biting / sucking), stimulating (bright colors / lights / sounds / movement), calming (repetitive / quiet). These are just some descriptions, but it can be so much more.
Our son’s needs are light tactile pressure with repetitive motion and then strong resistance chewing when concentrating. Some of the things that would happen before we figured this out was he chewed on lots of hard things like cords, plastic cups, and toys. And with his hands he would twist the edge of his shirts, poke pencils through his clothes / fabrics and cut fabric and his hair with scissors. We did the experimentation at first but it was expensive to have him end up not liking them and then we looked closer and noticed the similarities of his actions and narrowed it down. He has a few fidgets that he uses a lot now and that help him focus and not stim with dangerous items.
I hope this helps.