Just got back earlier this month from WDW with our four grandchildren (ages 1,4, 5 and 5). Do you have an idea of what your son likes? Our 5 year old girls were totally opposite from each other - one loved Space, Splash, BTMR and 7 Dwarfs...the other thought Frozen at Epcot was a bit "wild" so didn't want to try any type of coaster at MK.
I wouldn't use a FP for Hall of Presidents. I love the presentation but it runs every 20 minutes on a rolling admission and there really is no bad seat. I found FP at shows (at Hollywood Studios) just caused us to hurry up, get there and then wait and wait for entrance when we could have just walked in 5 minutes before the show started and gotten seats that were fine. No more "show" FP for us
Our 4 year-old and 5 years old who loved coasters both thought Barnstormer was too jerky...much preferred 7 Dwarfs. All three bigger kids loved Tomorrowland Speedway. They liked Tom Sawyer Island but had a really long wait for the boat there so it was only doable because we had multiple MK days. I think it's wise to skip if you only have one.
We unexpectedly did CoP when we had a bit of a rainy morning. It gave us an indoor escape and surprisingly held the kids' attention.