I cannot figure out the normal structure of this sentence. "whatever work it was in him to do". It is a clause. What is the normal order if it is not a clause? Or can you split the compound sentence into 2? And what is the meaning of the phrase "in him" ?

The full context is below:
"Young men who have reason to fear that they will be killed in battle may justifiably feel bitter in the thought that they have been cheated of the best things that life has to offer.but in an old man who has known human joys and sorrows,(and has achieved whatever work it was in him to do),the fear of death is somewhat abject and ignoble."