If he saw anyone else biting their nails he could be mimicking them. Kids will pick up the very worst habits from others. The nail biting may be stress related. If this started around the time when you brought the new baby home there should be a little time set aside where it's just you and him. Read, relax, listen to some soothing music, if you pray in your family set aside some time with him to pray together. The one thing that I found that is interesting with children is if you bring up issues that you find disturbing they are more likely to continue doing what they want. Try to ignore the nail biting and focus on your time together. He is probably going to keep biting his nails if he knows you don't want him to since that is something that he can control doing and he knows that it bothers you.