So, if your child can sit still and attend to a science lecture away from school, but can't sit still and attend to a lecture in class, you and the school need to look at what the differences are between those two environments, so you can figure out how to make classroom environment more like the lecture environment.
Let's start with the seating issue. The auditorium chairs that I've sat in are usually pretty comfortable, while school chairs typically are cheap, light and stackable, not particularly comfortable or ergonomic. Since your son complains about his chair, has anyone at the school thought to see if a different type of chair would work for him, or if his expressed discomfort is due to low muscle tone or some other genuine issue other than "inattention"? He is complaining that sitting in his chair hurts. If he hurts, that is going to make distractability a thousand times worse. Sometimes just having a gel cushion or a pillow for the seat or sitting on a ball instead of a chair, or having a chair that is the right size can really make a difference.
Is the classroom dim, calm, and quiet, except for the presenter's amplified voice, like a lecture hall, or are there bright lights and visual clutter, movement, and/or noises such as fans, air conditioners, echo-y surfaces, noise from the playground or hallways or music room, etc. that may be making it more difficult for him to attend to the material? If the environment itself is too distracting, having headphone speakers where the teacher's voice is amplified directly to his headset and the classroom noise is somewhat diminished, and having a screen or study carrel that helps block out some of the visual distractions, or even having permission to wear sunglasses inside may help a great deal with his ability to pay attention. Also look at the seating arrangements: sometimes teachers think it is "warm" or "friendly" to seat children in a circle, or even facing each other, when to some kids it can be "annoying " and "distracting".
Another important difference between a public science lecture and his classroom may be the instructional level of the material. It is worth noting that instruction at the academic challenge level and presented at the appropriate pace, so the material actually captures attention, reducing the effort required to maintain attention continuously, is considered a reasonable accommodation for ADD.
I hope this gives you some places to start. FWIW, I would try all of these other interventions before starting on meds.
