How do I stop my 3 yr old DS from biting his nails

[bigknickersbigknockers]

DS is 3.6 yrs old and has the habit of biting his finger nails and also his toe nails. I have ignored this for some time but have noticed the nails on his big toes are looking red and sore. A friend has suggested painting his nails with stop and grow (or some other remedy to make the taste horrible)
It seems cruel and I wondered if there is an alternative not so nasty.

mix salt and water together in a bowl and then put his fingers in it..

will that tast bad and make him not want to bite them biglips?

it works for me when i used to bite my nails years ago.