Leanne,
Since MAA and SAA are classified by lab test results on a scale, theoretically MAA could become SAA between one lab test and the next. But Andrew may never progress at all. I haven't seen a separate formula for diagnosing pediatric SAA; a lot of AA patients are children. But the classification is less important than the treatment (if any) that's required, so asking for recommendations about what to do (watch and wait? treat?) is something to ask the doctor. If you watch the test results over time you'll want to observe any trends (a count that keeps rising or falling over time) because that's the sign that something is changing (better or worse). That's what the doctors watch for too.
Your son's cellularity is about twice the SAA range but lower than a typical youngster. I'd ask the doctors what that tells them.
The main evidence I think you should watch for is any new or changed symptoms your son experiences. It might be helpful to record anything you notice on a calendar so you can tell the doctor how long a given symptom has been apparent. If there are no symptoms to report, that's fine! But doctors would rather have specific facts than vague reports.
If you didn't already get
Your Guide to Understanding Aplastic Anemia from the AA&MDSIF, you should definitely ask for it
here.