Asthmatics sometimes go from wheezing to apneic remarkably quickly. I would not feel comfortable assuming this boy's life is not in danger. It's fine to work on that, while the patient is being treated. So if all else fails, we have the out of asking whoever brought the child in for permission. This does't solve the problem of the case where the minor comes alone, or with a barely-older sibling.
State laws vary.
In Pennsylvania, physicians are specifically allowed to treat any infectious disease without obtaining parental consent. This includes STD's, but also such things as pharyngitis and otitis. I would term acute bronchospasm with accessory muscle use 'life-threatening'. Death from acute asthma is far from unknown. Both from an ethical and medico-legal point of view, I would rather defend treating without parental consent than not treating.