What should my teenage son put on his nails to stop him biting them?

[CelerySeller]

They look terrible and he would like to stop biting them.
What is the best stuff to paint on to deter him from biting them?
TIA

Cyanide

I don’t think painting on something ‘nasty’ will ever work. It’s more about getting to the bottom of why he bites them, eg anxiety.

Yes we're trying to get to the bottom of why he does it but in the meantime he would like to try the bad tasting nail paint.
Any suggestions?

Mavala stop nail polish (it's clear) worked for me x

Stop’n’Grow from Boots?

Vick Vaporub.

Biting nails is often about a sensory input need. Nothing wrong with trying the revolting nail varnish, but it is less likely to work long term. Can he try squeezing his finger tips, play with fidget toys, take up knitting etc. I admit that last oneness unlikely, just mean that it fills a sensory gap.

Dandelion leaves

I remember reading an article someone got rid of the habit by hypnotherapy. I used hypnotherapy myself (for a different condition) and it worked. Hypnotherapy has been recognised by the British Medical Association since 1965