how can i stop my son biting his nails

my 6 year old son has been biting his nails for the past 6 months or so but recently i have noticed he is either biting or picking the skin around the nail i have asked why he does this and he says it's because he doesnt like the feel of the skin round his nail he bites it that often that his fingers are red and angry and sometimes bleeds he also has an angry lookin red welt on his finger and u can see the ammount of layers of skin he has taken off, what seems to be the problem and how could i possibly stop it or discourage him from doing it?

Many thanks for your help
clare