Stayed at WL twice when my son and daughter were as young as 2 and old as 6.
My kids especially loved the nightly electrical water parade, pool slide, biking to the campgrounds, then at the campgrounds the petting zoo and pony rides! They used to have a small playground and have a sandy beach that is fun to play at. All restaurants are great depending on if you want something fast or slow. We even had pizza delivered to our room one night. The boat ride to the magic kingdom is fun, too and you will probably be going there lots with a 5 year old.
We own at BWV and haven't made our first trip there yet, but kids wish we were going back to the WL!!! Have a great time! I know your son will love it!!!