en_tn_condensed/job/41/12.md

I will not keep silent concerning ... nor about ... nor about

This can be translated in positive form. Alternate translation: "I will certainly speak about ... and about ... and about" (See: rc://en/ta/man/jit/figs-litotes)

concerning Leviathan's legs, nor about the matter of his strength, nor about his graceful form

These are three things about which God will not keep silent.

his

The word "his" refers to Leviathan.