You're probably not going to get a clear cut answer.
General rule: a student, who is a dependent of his parent(s) has the same state of residence as his parents. If you file as a MD resident, he should also. But even that's not clear, in his circumstances. If you do not file a resident MD return (i.e. you file a non-resident MD return for the rental income) he's probably safe in not filing a MD return.
