off his guard vs off guard

off his guard and off guard both are adjectives.

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

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

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

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