it is actually possible, but very unlikely. It would have nothing to do with the age of the boy.
The youngest girl ever on record to become pregnant was only 5 years old, for reasons unknown she actually began menstruating at just 18months. This is a medical annomaly and pretty much unheard of.
Girls typically begin menstruating around the age of 10-13, but sometimes as young as 8. Once a girl has begun ovulating, it is possible to become pregnant (you actually ovulate 14 days before you menstruate, so even a girl who has never gotten her first period may be at risk for pregnancy because she may unknowingly be ovulating).
So yes, it certainly is possible for an 8 year old to become pregnant, it is extremely uncommon (and very sad) but it does happen.