I cannot imagine following the booklet is going to stifle creativity. If anything, it probably helps to improve visual/spatial ability and fine motor skills. He probably can also learn some basic building techniques for his own design in the future.
Sure, free build is more creative but to build anything recognizable may not be a skill he has yet at 5. And if he can follow instruction well at age 5 and that is a feat in itself. I even enjoy these sets as an adult. I found it meditative.
You may consider signing him up for a Lego class offered by a art center. They often teach how to free built by making a design sketch first, and go from there.
