Then he could attend classes at a local community college and earn a proper high school diploma. At least there he could test out of most classes until they determined which ones he still needed to take to get enough credits to graduate.

Although I’m pretty certain that both the US military and any local fire dept would accept a GED in place of a high school diploma.

Edit: yes, any branch of the US military will accept a GED.

Edit 2: my local fire department will accept a GED at minimum, and I live in a medium sized city.