Lake Como or Cinque Terrr?

Lets start from the end of your trip. Since you depart from Milan you must spend you last night in Milan or near Milan airport because flights back to North America a depart in the morning and you must be at the airport at least 2 hours prior to your flight. Therefore you can use the day before your flight to see Milan.
You can spend the first two days wherever you wish: lake Como, cinque Terre, Florence, etc. that is really your personal preference.
Are you renting a car or using public transport?

You will likely land in the morning and not be able to check into a hotel until later in the day. We usually travel to our first destination on that first day; no sense just hanging around waiting for the hotel room. We love Lake Como so I whole heatedly recommend it.

There is also Venice. A lovely and unique place to get over jet lag, just 2 hours train ride from Milan.

The Lake Como area is just about an hour by train from Milano, great place to recover for a day or so. Take the ferry to a couple of the towns on the lake. Or pick up your car at the airport and drive there directly. Note: this area is north of Milano, so it will add an hour to your trip to Umbria.

You could go to Verona instead, but the lake is so beautiful and relaxing.

There's lots to do in Milano, your son might enjoy the Duomo roof (elevator to the roof) to get close to the gargoyles, and the Castello Sforzesco which has a museum and beautiful grounds.

When you land take the train straight to Lake Como area and spend your 2 days there. Nothing like the lake for beating jet lag. It's a quick hour train ride.

Thank you all for your comments. Much appreciated. Has anyone visited Cinque Terre? Was it worth the trip?