If it starts to look like adult school is an option, some questions to ask include:
Who issues the eventual diploma? (in some places it is the original high school or another local high school, in some places it is the school districts, and in some cases it is the state board of education). Some students really care about this, others don’t.
What classes are offered at the adult school? If he will only be one or two classes short, can he plan his regular school year to take things he’s really interested in, and then just pick up the leftovers in adult school?
Are there any classes he doesn’t need to take if he is going to get the adult diploma? For example, some states won’t require phys ed.