I'm not sure about stress - but he is certainly experience tension (and this is being expressed phyically as he bites his lips). At 3, he's a bit young for talking therapy! I'm not even sure if he needs any kind of therapy, although play therapy or art therapy might help him if he is in fact holding negative emotions inside. These can come from moving houses, changing schools, dealing with siblings, experiencing a rough teacher and so forth. However, before we even go there, I'd suggest that you try Bach Flower Remedies. The flower "agrimony" may be useful. A consult with a Bach Flower Practitioner would be even more helpful. But if you can't arrange that, just go top a health food store and get agrimony. Put 2 drops of it into a small amount of water, juice or milk, 4 times each day. Continue for a number of weeks and if you see that he is beginning to relax, just continue until he is no longer biting his lip.   Good luck! Sarah Chana