If he needs only a class or 2 to graduate, consider online classes.
The GED is easy to get and if he ends up going on in his education and getting any sort of degree or certificate, no one will care. By the time your over 21, you probably don't need any reference to HS level education on your resume either way.
One sticking point: If you're considering military service, you MUST have a diploma. No GED accepted.