Continual means start and stop (like raining; it happens frequently), while continuous (unlike raining; it ends eventually) means never-ending.

Continual: things that continue for a period of time, but there are interruptions in the process - like arguments or rain: My mother continually tells me to clean my room. / My friend and her husband argue continually. Continuous: things that never end; things happening without interruption - She cried continuously for hours./ Rivers flow continuously. "He studied continuously for 8 hours." - He studied for 8 hours without taking a break.

Whoops, I forgot to answer the question. Neither option would be correct. I would prefer to say "He continued studying German even after graduating school."