I'm not sure of exactly what answer you're looking for here or if this is suppose to be an English essay, but I'll give it a try.
Your friend could try to calmly talk to his parents and explain why he feels he shouldn't pursue a career as a doctor because he isn't good at science.
Also, he could show them the good grades he gets in English, History, and Geography and explain why he should pursue a different career that has to deal with those subjects that he is good at.
Prove to them that a different career would be a better option and maybe he could even get his teacher to vouch for him.