Yes, able to be done in a day, but a full day.
You need a ticket to the Vatican Museum, which includes the Sistine Chapel at the end of the tour. St Peters has no entrance fee, and someone else can chime in, but I believe you can exit after the Sistine chapel and go directly into St Peters as opposed to exiting and heading all the way around into St Peters square and standing in line, though in several trips we have not had to do this. There is a separate line to the crypt below St Peters where the Popes are interred, and a separate line and fee to climb the Dome (Which your Son may really enjoy), as well as completely separate reservations for the Scavi tour and the Gardens tour.
As for other sights, no ticket needed for the Pantheon (the Parthenon is in Athens, Greece) and you could hit that plus Trevi fountain, Piazza Navona, Spanish Steps, and Camp di Fiori on a walk from St. Peters to the Colosseum (see the Rick Steves Rome walk). It would probably be as easy to buy a ticket for the Colosseum at the entrance to the Forum (a combo ticket), walk through the Forum, then to the Colosseum. There are some other tours that cost extra and need a reservation at the Colosseum, but for the time you have, just do the regular ticket.