I can't recommend Diana Wynne Jones highly enough.
Her books are better than Harry Potter in so many ways and she has SUCH a prolific imagination—I have 32 books of hers on the shelf next to me, and omitting the three books of short stories, they are set in 17 completely disconnected "universes." Most are standalone or semi-standalone; I did a tally just now and there is a series of 6 books, a series of 4, one of 3, and two of 2. And the works in the series are really standalone as well. (Except for Book Four of the Dalemark Quartet—you will cheat yourself if you haven't read the first three. But it doesn't really depend on them even then.)
Her works are all original and unique—her career spanned from 1966 steadily through the decades to 2010, interrupted by her passing in early 2011 at age 76.
If your son enjoyed the early Harry Potter books, I would say start with The Year of the Griffin—a wonderful, joyous story that I would say does an even better job of showing the wonder of an academy of magic than J. K. Rowling ever came close to. (And I've read the entire Harry Potter series several times and loved it. But DWJ is a league of her own.)
The Year of the Griffin is actually a sequel, but don't let that worry you. As with most of Diana Wynne Jones's sequels, it is another story set in the same world following different characters (new characters and some who were secondary characters in the earlier book), rather than a continuation of the same story.
Another good starting candidate is Power of Three.
Or Howl's Moving Castle. Or The Lives of Christopher Chant. Or The Homeward Bounders.
She does have a couple books not suited for young children. Avoid: Deep Secret, and A Sudden Wild Magic—but read them yourself!
Neil Gaiman (author of Coraline, Stardust, American Gods), who was a good friend of hers until she passed away, said her books are a joy to read aloud—I may dig up the quote later, but he said her books unpack very tightly with scarcely one word wasted in a thousand, and reading to yourself it's easy to miss things without realizing it.
Edit: Found the quote.
One of the privileges of having kids is that you get to read out loud to them, and reading Diana out loud is a delight. Her books unpack very tightly, with scarcely a word wasted in a hundred thousand, and reading them silently it's easy to miss stuff without knowing it.
—Neil Gaiman, September 2002
The comments on this blog post are also filled with people singing her praises.
Her characters are SO real and SO vivid, and her writing SO excellent, she has no rival in the field of children's/young adult fantasy.