Son chipped lower left molar, a sliver the width of tooth & square. no pain, swelling, bleeding, but feels it when breathes thru mouth. emergency?

Chipped molar: Have it checked by your dentist. It may be a small chip, or there may be a larger fracture, especially if there is a filling in the tooth. Sensitiviy to air (breathing) indicates that a deeper par of the tooth is now exposed.

Serious: Tooth is severely damaged and at risk of becoming extremely symptomatic. Your son has a problem that requires immediate Dental (not ER) attention. Like any fracture, this one needs care. Please call your Dentist now.