Which would be a better extracurricular activity for a hyperactive boy?

How old is he? If he's young, then sports are definitely the way to go - even 'normal' boys who are in grade school sometimes have trouble dealing with all the effort, concentration, and coordination (and some people just never learn the last part) that goes into playing an instrument. Since it's only 'for now', I'd say sports, he can always learn an instrument later, and there's nothing to prevent him from eventually doing both. In fact, in 7th-8th grade, most schools require kids to take some sort of music cles, and they're either required or strongly encouraged later in grade school, so he'll learn an instrument eventually.