My son is 6 2 months ago he like lost his 2 baby teeth one grew in quick the other one has not came threw the gum yet. why? should i be concerned?

Next tooth..: Very little in your body happens together - your body is not, contrary to popular belief, symmetric. I doubt his teeth popped through on the same day when he was a baby, and it's unlikely incoming adult teeth will follow an identical timeline. If you are having routine dental care for your child, your dentist will notice abnormal eruption, so i wouldn't worry about it and see dentist regularly.

Antimere: Antimeres, same tooth on either side of the midline, if in bottom front should erupt within a month of each other. This may be a sign of overall arch length insufficiency. Please see a growth and development specialist, an orthodontist, for advice. The timing problem you are describing is a sign that your child's dent-facial development should be evaluated now.