My son has never grown one of his lower incisors, he only has three, is this normal? should he be concerned?

Not normal: But, depending on a host of factors, may or may not require treatment. Get an orthodontic perspective and make sure the tooth is really absent and not impacted in his lower jaw.

Partial Anodontia: Missing tooth or teeth. When there is one eruption anomaly there are often multilevel eruption anomalies. If a tooth is missing in one arch the tooth-size ratio to opposing arch is incorrect. Please see an Orthodontic Specialist, the Dental Specialist that is trained to provide Dento-facial Growth & Development Supervision.