off his guard vs off one's guard

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

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

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

Other synonyms of off his guard include napping, off-guard, off guard, off her guard, off your guard.

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