Are there any good nail strengtheners?

My eldest has a habit of biting their nails.

My hands are in such a mess right now that I can't even look at them.

I've tried various methods, but

He said his fingernail broke.

I had no choice but to clean up, and now they say they are biting.

Of course, it doesn't make sense, but

Since he says he thinks that way

From my perspective, they applied something like a nail strengthener.

The phenomenon of fingernails breaking or splitting

I want to reduce it.

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

Please recommend some good products.