When should I exchange nouns to pronouns?Is there a obvious rule in it?
There is no rules, but Chinese is also try to avoid duplication

父亲今年65岁了,他以前是一名律师。他的爱好是旅行，所以他经常去旅行. is totally OK.

Sometimes you can't, then try to avoid continuous duplication.

父亲今年65岁了,他以前是一名律师。父亲的爱好是旅行，所以他经常去旅行.

Author prefer synonym to eliminate duplication.

父亲今年65岁了,他以前是一名律师。老爷子的爱好是旅行，所以他老人家经常去旅行.