Anything Cars, Thomas, or Elmo. They also have great apps online for download. My ds1 is 3, but can adapt the older games to suit his needs. He may not get it, but he has fun. So your son will have no problems.
Ds1 is playing with it now or I'd be more specific.
We have a few games (cartridges and ones that were downloaded). My 4.5yr old is currently into Phineas and Ferb (cartridge), Penguins of Madagascar (cartridge), Splurgle (download), Roly Poly 1 and/or 2 (download), and he loves the Rescue Bot videos (download). I know he is into other games (cartridges) but cannot think of them right now.
Re: Leappad games 5 yr old
Ds1 is playing with it now or I'd be more specific.
I would suggest- leapschool Math and the Reading games- There is also a Dinosaur game that my daughter really likes.