
What you are describing bottom jaw shorter than top jaw would be an OVERSHOT mouth. Now if the gap is slight then it may come right as the lower Jaw grows for longer than the top jaw.
An undershot mouth is the type that a boxer or bulldog has where the bottom jaw protrudes in front of the top, and is normal for most of the shortfaced breeds.
If the fault is slight then it shuldn't cause him problems, but you won't really know for sure until he gets his second teeth.