Do baseball rules allow a eighth grader to play a seventh grade game?

Can't say for sure but most leagues determine teams by age in the summer and grade in school. There can be a combination in school that would allow a very young 8th grader to compete at the 7th grade level. If the kid is a stud he should be playing 8th grade school. If he is a dud because of size wait til he grows and keep at it. If he is a dud not because of size then find another sport! Baseball is BORING when you are not good and you know it.