This statement expresses in a deep manner something about his nature.
What would be a proper translation of this phrase. The attempt looks like this:
Это высказывание, глубоко выражает словами кое-что о него сущности.
I would rather rephrase it as
Это высказывание даёт нам понять нечто важное о его сущности
(lit. "This statement lets us understand something important about his nature").
Not exactly the same but sounds much better and get the message across (I hope) approximately in the style you are trying to mimic. Maybe even more bookish I am afraid that there is no way to keep "in a deep manner", though you may link it somewhere else (i.e "expresses something deep about his nature", "expressed something about his true, deep nature"). Note also that "something" is one of the sources of the problem here. As it does not sound bookish and profound to just us "что-то о его сущности", and if fact the whole "tells something about him" sounds kinda informal in Russian, it limits the degree of freedom you have here. You just have to juggle the words around to make it sound right.
Could you clarify what is it exactly you are writing?
"о его" is used without the initial "н". Here "его" is a possessive adjective (his), not a pronoun (him), though, obviously, the possessive form originated as the Genitive form of a pronoun.
To refer to the somewhat downplayed "in a deep manner" part, something like this might also be appropriate here, depending on what is the focus of the original statement (i.e. it is always better to provide additional context for such translations):
"Глубинный смысл данного высказывания позволяет лучше понять его сущность" - note that this is quite formal and to be used in written form or in official public speeches.
Another remark - the translation of the last word may depend on the context, it might also be translated as "характер" or "природа" (less probable but possible) depending on what exactly is to be understood about the person in question.