Missing Grey Tabby Male Cat, 3 yrs old. Help!

Grey Tabby Male, 3 years old, neutered, microchipped. Many has thin stripes but also a couple of spots, has a round baby face, legs are quite short and he’s quite podgy and has a light pouch on his belly. Indoor cat who only goes into back garden, never has gone over the gate but decided to but I have no idea where he is. A few potential sightings on Banbury road and flats opposite. Tried everything to attract him back home but have been unable to, it’s been 8 days now really losing hope. He’s never been out like this before, most neighbours have checked sheds and some have allowed me to check. He is really friendly but cautious, gets nervous and is very attached to me. Been calling out on the streets whenever I can but no response. He’s sometimes vocal but rarely meows. He has a sister at home who really misses him and wants him home.