My 4.8 yr old DS is biting his nails

[Fruitpastels]

Not sure where to post this. My DS is biting his nails and the skin around the nail. It looks so sore . I've not really seen him do this, so I'm shocked to find his fingers in this state. Not sure how to the tackle the problem. He goes to pre-school and starts reception in Sept. I can't keep my eye on him all of the time. Any help would be greatfully received.

Once you found the answer, please post it. DD started biting her nails when she was about 5 and hasn't stopped. She is 9.2 now. She bites her finger and toenails and picks the skin around them despite me offering bribes, rewards, payment, punishment for her to stop. She often wants her nails painted but there isn't enough nail to paint!