The current hypothesis: nail biting helps even out our emotions. When we’re bored, it provides stimulation; when we’re stressed out or frustrated, it provides a temporary calm.

I would have suggested Stop n Grow.

There is stuff you can buy to paint on fingernails to stop nail biters, it tastes nasty! Maybe you could use that on your sons fingers? Or maybe there’s something similar for fingers

This link has some suggestions to help stop it.

https://www.calmclinic.com/anxiety/symptoms/nail-biting

I know personally I find keeping my hands busy helps. I tend to paint or do other crafts to keep busy. I sing when I drive to lower my anxiety, as it ups happy hormones. But for a kid, things like fidget spinners and similar toys, or maybe chewable jewelry, may help keep hands busy and away from the mouth, breaking the habit.

Hubby and I both do this, though not to the extent in this link. It may be linked to anxiety.
https://en.m.wikipedia.org/wiki/Dermatophagia

Yes you can still buy Stop’n Grow https://www.yourchemistshop.com.au/stop-n-grow-biting-deter-2.html?CATARGETID=620011870000498395&cadevice=c&gclid=EAIaIQobChMIjdXjv7us2wIVTg4rCh22mweZEAQYASABEgLtQvD_BwE/
And there are many other products like this on the market. Just google ‘stop biting fingernails’ and you’ll find a whole variety of products and tips.

There used to be a product called Stop and Grow but I’m not sure if its made anymore. I know people who have used it to stop them biting their fingernails. It may help?

My 6yo son also has this problem
You can get chewy necklaces or other chewing items such as pencil toppers
Try googling
Therapy chewing devices
Good luck