P.M.
Nail biting is often a response to anxiety. I'm not a fan of the aversion techniques because that does nothing to help him release this anxiety. You should strongly consider offering him another, more suitable solution.
My son was also a nail biter. So much so, that he'd often chew his nail until he was crying (b/c he chewed it so low it was too short & was hurting). For my son, things that are crunchy are very soothing & relaxing for him. Crackers, pretzels and chips work well but his favorite is dry cereal... Cheerios, Kix, Life. The more resistance it can offer, the better. We also offer him other chewy things - Chewelry, Tri-Chew, and his favorite Chewy Tube. These are also socially acceptible alternatives to pacifiers. My son is 3 and we also recently introduced him to chewing gum. Be extremely careful if you choose to offer that. My 2 yr old daughter does not yet understand that she needs to spit it out when she's done.
As recommended earlier, you may want to discuss this w/ your pediatrian. He/She can help you select an appropriate solution.
Good luck in your search,
P.