My son was circumcised at birth. Looks like the frenulum is now missing. Is that common. Will this cause sexual problems later in life?

Frenulum. Yes this is common after circumcision and no it won't cause problems later.
Poor circumcision. Maybe ur son did not have the circumcision done properly. Talk to the docotr to see if there were any complications or too much of the foreskin was removed and the frenulum was also accidentally removed.