Dr. Seuss is a favourite at our house too. But hey, each to his own, right? If your son loves the rhythm of words in Dr. Seuss, he might like some fun poetry. You could try Shel Silverstein, or again, A.A. Milne (When We Were Very Young, and Now We Are Six.) My kids love and have memorised a number from those two books.
Or for a bit of nonsense apart from Dr. Seuss, try some of Robert Munch's stuff. Most kids love it. Easy, fun reading, but nothing wrong with that once in a while.