My baby had his first tooth seen at 6 months (fully breast fed). The first tooth can be seen ranges from 3 months to 12 months. Don't worry. You will see it sooner or later.
Don't worry! I think 遲出牙 is better when it is within the normal period (4-12 months). It is because it would minimize the opportunity that he bite you, and thus prolong the period of breastfeeding.