Not enough talk about his thumb here, IMO. I used to bite mine when I was a kid. In my early teens I decided it was enough, and so everytime I had the urge to bite, I just went and got some clippers and cut them instead. It fixed me forever. No urge to bite. My bite was related to the annoying feeling of having long nails. But I also told myself I’m not allowed to clip them past the white part into the pink. So that also helped regrow them perfectly. Highly recommend to try this method. Very simple and won’t cost you anything but some clippers.