My 3 year old bites his nails constantly. I keep them short so he doesn’t feel the need but any time there’s a very small amount of growth or a hangnail he starts chewing. They are so short and look sore. I’ve given him a nail file in hopes he would use that, I ask if any are bugging him and I’ll help him with them so he doesn’t have to bite them, I’ve told him he needs to stop or they will start to bleed.
I use to bite my nails constantly, up until I was 17, 18 years old. Never once let them grow at all. What got me to stop was keeping my nails painted. I hated the taste of nail polish so I kept nail polish on my nails at all times for 2 months. All it took was 2 months to break the cycle. I’ve been growing my nails out ever since to the point they break from how long they get. Maybe try painting his nails and see if that works, they make kid nail polish, never had it cause issues with my daughter she just turned 4.
Nail biters polish. Someone told me garlic works too. Idk how I feel about the garlic method. 
Get the nail biters nail polish. It has a nasty taste to it so it deters them from doing it. It’s clear and is harmless. Just tastes bad.
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Get a no bite spray or nail polish it tastes bitter
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When does he do it? Most nail biting is a reaction to anxiety of some form. Bored, anxious abt something, tired, scared, etc. See when he does it most & change his view, so to speak.
I bit mine for YEARS. You cab try the Bite No More polish, even hot sauce & nail polish. Ultimately, he has to choose not to.
Find a male he’s close do & have them show him “pretty nails”. Encourage him to have nails like them. Reward him when he doesn’t do it.
The biggest thing; find out why & correct it there. 
I told my daughter that I wad going to take her to the doctor and I was going to have him to give her a shot in.every single toe. Cuz she could and would get a infection In your case fingers .