Wow, I do that too!
It's best to find out why he's doing it and then work from there. Is he nervous, anxious, or concerned about something? I have been a "picker" since I was a wee thing - and I tend to do it more frequently when I'm stressed, worried, nervous, or anxious about it. Most of the time I don't know I'm doing it until someone says something to me. It puts me in a "zone" where I'm really focused on picking my nails and nothing else around me. Maybe your kiddo is in a zone too, and you can ask him when he's doing it, "hey kiddo, why are you picking? what are you thinking about?"
and see what he says. If he says "I dunno" like a lot of kids might, ask him if he's concerned, worried, upset about something (you pick what word he would respond to best)...
And not to freak you out, but if he picks at his nails it might be possible that he picks at other parts of his body like scabs, hanging skin, maybe ingrown hairs and eventually pimples. It's part of that compulsive picking thing - there's actually a term for it - go on wikipedia and look up "dermotillomania." That's what I have. Really the only thing that helps reduce me from picking is the awareness that I'm doing it, and someone to talk to about things that are going on in my life.
Please feel free to message me if you have any questions... Maybe I'm totally wrong, but if he continues to be a "picker" don't be too alarmed.