off guard vs off his guard

off guard and off his guard both are adjectives.

Word	Noun	Adjective	Verb	Adverb
off guard	No	Yes	No	No
off his guard	No	Yes	No	No

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

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

off guard (adjective)	off his guard (adjective)
not prepared or vigilant	not prepared or vigilant