I doubt you will find much you can get into on Easter Sunday but there is still plenty to see and do! Just start walking! Rome is a magnificent walking city. Remember, if you get tired of one century all you have to do is go around a corner and you're suddenly in another century! Your Monday schedule seems a bit light especially since all three items are quite close to each other. If you miss something on Sunday, you'd be able to catch up on Monday without any problems. Trevi Fountain is under rehab, so there isn't much reason to spend a lot of time there. The Pantheon is my favorite for the afternoon or later - as wonderful as the building is, the piazza is a destination unto itself! Find a table for a drink, snack/dinner and watch the people. Head over to Piazza Navona and people watch there with a gelato in hand (after dinner is the best time). Spanish Steps? Aren't they under rehab too? I don't particularly find that area all that interesting so I'm not the person to ask. Check out Trastevere for wonderful churches to visit, walks, restaurants. St Peter's Square is always open and can be a great place to sit. The museum is amazing but a 13 yr old might find it sensory overload. He probably will be happier at Pompeii.
Orvieto is an easy day trip from Rome and is a very pleasant place to visit. Lots of shopping, great local wines and foods. Your son might like the 'underground'. The Duomo isn't as awesome as the one in Siena, but it's worth a visit.
Sorrento? If it's warm enough, your son might like to go swimming while you do some shopping. Also, take a bus or boat from Sorrento to Amalfi Town and then return the other way. Bus is exciting, boat is relaxing and you see more at the same time.
Have a wonderful trip!