Anti-bacterial shampoo and powder do not work. Putting socks on the foot turned out to be a no-no (and funny, the more he chews). I read about washing it with tea but that would cause it to turn brown. Putting something bitter is not an option I am considering.
I am at a loss on how to handle it. There's a wound. Callouses have formed. My oh my!
I have observed he is into this activity when he's alone doing nothing, kinda bored possibly and after bath.

Finally, I resorted to putting this shield after his bath so that his mouth won't be able to reach his paws.
The fur on his paw started growing again, the callouses reduced in size and the stain is not as dark anymore. However, he still chews so he has to continue wearing this "thingy" after bath.