nailbiting in 3 year old

[nixpix]

My DS is almost 3 and has been biting his nails for around 6 months. He frequently bites them so much they bleed. I'm worried he will get an infection not to mention all the bugs that lurk under his nails. (Although obviously we do wash his hands!)
I really want him to give up this bad habit, but am at a loss as to how to get him to stop. I ask him to stop when I see him doing it, but then as soon as my back is turned they're back in the mouth.
Any ideas??

i have 4dc. dc1 used to bite his nails but only when he got to about 10yrs old. We stopped him by putting his favourite pack of sweets on fireplace. At end of week if he hadn't bitten nails and they had grown to show white bits he was allowed sweets. If there were no white bits which meant he'd bitten them then i got his pack of sweets. Worked a treat! dc3, aged 5, has always bitten her nails. Can't find anything that works with her so i just don't make a fuss of it now.

Have you tried that stuff you paint on?