Well an overbite is when the upper teeth extend abnormally far over the lower teeth. Your boyfriend may only have a slight overbite so you would not have noticed it. Either way as long as he is not concern about it then it is really nothing to worry about. If he does not like the overbite he has, then it can be corrected with braces.
You also may not have notice because you are not the type of person to look for flaws that one may or may not have. If only more people were like that.