Going back to your first question about full FN or just one level...
if you keep the middle in he can have the full FN. If you want to open up the cage into one open area, you'll want to be very careful about shelf placement. Babies can be really crazy and/or klutzes and are known to climb high and let go just for fun. a drop that's too long can cause injury of even death... so keeping the levels split is way safer until you know he's not crazy or a klutz.
just remember it's not the total area, it the length of the longest possible drop that matters. if you lay your head on the bottom of the cage and look up and can see the very top from any spot - the chin can fall that far and you need to add something like a shelf or fleece hammock higher up to break a fall.