My 7yr old son lost two teeth before christmas and only one has between back. the other has not. he has lost several before and they are fine. why?

When they are ready: Teeth have no real time line for eruption. They erupt when ready. The permanent tooth that has not erupted could be at an angle and not erupting as fast or possibly even trapped by another tooth blocking its eruption. The best thing to do would be to have an x-ray taken to see the exact position of the tooth. Consult with your dentist.

Needs evaluation: When there is one eruption anomaly, there are often multiple eruption anomalies. Your son's 7, with an aberrant eruption pattern. Please see an orthodontic specialist (growth ; development expert) for initial examination (often complimentary) to make sure everything's ok. Ask your dentist for a recommendation.

Early Orthodontics: Every child's development is different. It is possible at this point to determine whether there is adequate room in the mouth for all of the permanent teeth. There are some conditions that are much easier to treat if they're caught at an early age, when a child's natural growth process are going full speed ahead. See an orthodontist for evaluation.