off one's guard vs off his guard

off one's guard and off his guard both are adjectives.

Word	Noun	Adjective	Verb	Adverb
off one's guard	No	Yes	No	No
off his guard	No	Yes	No	No

As adjectives, off one's guard and off his guard are synonyms defined as:

Other synonyms of off one's guard include napping, off-guard, off guard, off her guard, off your guard.

off one's guard (adjective)	off his guard (adjective)
not prepared or vigilant	not prepared or vigilant