The 2 lower center incisors are usually the first to go between the age of 6 to 8. Then the upper ones follower before others. These days, I noticed that kids loose their first tooth as early as 4 or 5 years old. Children whose baby teeth erupted early usually lose them before late teethers. I don't think it is unusual though for him to change a different set from the regular pattern because some babies do not grow teeth in the usual pattern. You may want to see a pediatric dentist to confirm.