Florence can be considered hot in July. Temperatures are between 20-30 degrees Celsius and humidity has a high percentage (about 60-70%). So we can say it's really hot.
Even though Norway is not in the European Community, thanks to a special agreement, your 16 year old son can have free admission to most museums in Florence. The Uffizi Gallery, the Academy Gallery, Bargello Museum and all State and local museums are free of charge for EU citizens under 18 years old. The Cupola or Giotto's Bell Tower would still require an admission fee.
I recommend he carries his ID with him at all times since they'll ask for it when purchasing tickets. He still needs to go to the counter and get a ticket marked "free" admission to get in.
I hope you'll share your experience with us when you're back in Norway and tell us how your vacation in Florence and Tuscany turns out
Hot is relative to each person, depends where you are from. I personally love the summer and don't consider those temps for July "really" hot but just right What I don't like is high humidity, that is another matter.
But considering you are coming from Norway, probably all of the Mediterranean is a hot place