My son did this when he was about that age, and I bugged him, it didn't help. He gave up on his own within a year.
My middle DD never bit hers, thankfully!
My youngest was a horrid biter for a year or so, she had her nails down to basically nothing, bleeding. But lately, she has stopped for the most part.
I am a nail biter, but if I catch myself I can stop it. It's absentminded chewing while reading or watching tv that I have a problem with.
Perhaps if you just wait a bit, he'll stop on his own? My two did. good luck!