I would get involved in things he liked and use those for after school activities. Don’t forget a curios, day dreamer grew up to be Steven Spielberg thanks to his mother who recognized his interest and bough him his first video camera. She also helped him cut classes so they can shoot movies together, I wouldn’t go that far. But a creative kid needs an outlet and if the school can’t provide it it’s up to the parent to provide support.
Very often when children daydream, they are imagining things, inventing things in their minds, experiencing quiet time within themselves. This isn't a bad thing. A child could be staring out of the window in school watching a leaf fall to the ground. The child could be counting the seconds until this leaf falls to the ground, questioning why the leaf is taking so much longer than say a rock, inventing an airplane that can fly for miles, etc. Your child is using his intelligence to his advantage. Somehow the lesson in the classroom is not grabbing his attention. Find out when he daydreams the most. Which subjects, is it during a time when the teacher is talking or showing visuals. Possibly if the teacher is talking and he is not an auditory learner - he will tune out. Observe him one day in the classroom and make notes. You can pretend like you are there to help the teacher (staple papers together, correct homework, etc.) but really you are doing and observation as to what kind of a learner your son really is. Sneaky - but very effective!