Definately, natural sunlight is the best for your chameleon!
However, you have to keep a constant eye on it, because chameleons have a tendancy to explore and get lost! And I read enough of these sad stories in the past! :S
Another important thing is to offer him constant watering and constant shading if he needs to. Just run a dripper and put him on a small tree or somewhere where he can either bask directly under the sunlight or covered by the leaves!