Do you take care of your nails?

My eldest has a habit of biting their nails, so their hands are in such a terrible state right now that they are unbearable to look at.

I've tried various methods, but

He says that his nails broke, so he has no choice but to bite them while trimming them.

Of course, it doesn't make sense, but

Since he says he thinks that way

From my perspective, I want to apply something like a nail strengthener to reduce the occurrence of breakage or splitting.

Is there a nail strengthener that you apply like nail polish?

Please recommend some good products.