Be honest here--my 12yo son has his spring break the last 2 weeks of March. Is it worth planning a trip to do the fjords and northern lights during that time of year? I assume weather will be cold and wet most likely. We are willing to brave that if the destination is worth it but we don't know much and are looking for guidance. TIA!
To be honest, late March is very late in the Aurora season. You might see them, but the odds are not what I would base a transatlantic holiday on. But the fjords are there and southern Norway will not be that cold in late March (although I guess it depends on what you are used to).
March is not too late; the lights happen any time of day, any time of year, if there has been a 'solar flare' from the sun that has intersected with the earth's orbit. However, they can't be seen during daylight or when there are heavy clouds at night. Winter brings longer nights in the northern hemisphere which is why we associate the Aurora with winter. The transitional months of March and Sept still have long enough nights, better weather than the deep winter, and for a photographer it's a marvelous time to photograph the lights because lakes tend to be no longer frozen over and therefore provide marvelous reflective surfaces.
I highly recommend that you try looking for the northern lights in Alaska during that time. There's lots to do and see in Fairbanks that month. I have seen spectacular Aurora up there in March. The weather is cold but not impossible - not much below zero and is easily tolerated with proper clothing and footwear. Your rental car will have an electric plug to use at your hotel to keep the engine from freezing.
You will enjoy the World Ice Art Championships in Fairbanks. which runs through to the end of March and is a great thing to experience. Also, the Open North American Dog Sled Race happens in Fairbanks on the 2nd weekend next year and it is a blast.
Anchorage is another gorgeous destination but since it is on the coast it tends to have cloudy weather and is less reliable for clear skies for aurora viewing.
Iceland is another popular destination fro aurora viewing but I have not been successful during my attempts there due to cloud cover. It's a wonderful place to visit but I have to say that my boys, at the age of your son, got more inspired by Alaska than Iceland.