The books and the movies are quite different. The film-makers eliminated a lot of the content of the books, and then added other things that are more do-able on film than on printed pages (as film-makers usually do).
I greatly prefer the books myself - they're much richer. So I recommend that you start with the book (reading it aloud to him) - and *maybe* then get out the movie, explaining that it's different from the original. He can even look for the differences. Think how different the Disney "Frozen" is from Hans Christian Andersen's story, "The Snow Queen." If it means your son waits for H. Potter for a while, it isn't going to hurt him a bit.
On the other hand, watching the movies first often leads to less or no interest in the original books.
As the books continue, they are less and less for seven-year-olds, though. So please space them out a lot.
P.S. The "Charlotte's Web" book and movie are pretty close to one another - maybe not like sisters, but at least like cousins. Another thought: When my kids were in their teens and were assigned classic literature in school (Shakespeare, Dickens, Austen, etc.), I found it *sometimes* a good idea to find them the film version closest to the book. The reason seeing a film first helped was that it helped them comprehend the basic story without having to learn the style first. Once they knew what it was about, they found it easier to read and love the real book. I don't know if that applies for a seven-year-old, though!