Hi, I am not an occupational therapist. However, you may want to try the following to strengthen his fine motor skills sustainably.

1. get him to play with play-doh or Putty often
2. if he likes legos or connector pieces game, get him to use them often
3. buy crayons of various sizes - big, small, rectangle, candle or teardrop shapes
4. handwriting without tears is also a great place to support his fine motor and writing
5. get him to write, draw, doodle, or paint in a standing position
6. rock climbing - if he likes it. As for safety, you may want to choose an indoor play structure so you have more control of his whereabouts while he still gets his upper body and fine motor workout

I hope this helps.

I recommend exercisers that work on the individual fingers, like grip masters. https://prohands.net/
Here is a website, just to give you the idea of what I am talking about. Musicians use these to help strengthen individual fingers. You can find these at most local music stores or Amazon, too. Many of my clients use these as fidgets, too, because you can caryy them with you, and they make very little noise.

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https://prohands.net/

Is he in occupational therapy?