K.C.
Little People toys are great at this age. There are so many to choose from so you can find ones that fit the things he's most interested in.
Blocks of any kind are also great. My daughter (20 months) just got some megablocks as a hand me down and she is loving them. My 5 year old is playing with them too! Lego Duplos are good too, especially a basic set that just comes with bricks and doesn't make a specific item once they're built.