...the exercise gives the translation of this sentence as 'We are astonished at his good marks' ...however, I don't see how you can tell the gender of the person with the good marks from this sentence...so surely the translation should be 'We are astonished at his/her good marks' ...? Unless you can tell the person described by the verb 'étonnons' ...but then I am sure this has no connection to the person being referred to in the sentence...
Look forward to your answer...