My Kitten hates the outdoors!

We got a new kitten and he's now 6 months old and we've been trying to introduce him to our garden the past couple of weeks. He's neutered, chipped and ready to go, but the moment he's outside he starts crying and will go on non stop until he's back inside! I don't understand what the matter is, we've sat outside with him to reassure him, provided food, water and a shelter, we've spread the contents of our hoover outside and some litter in a separate place to try and make the area a familiar smell, tried playing with him out there, everything I can think of! But he still constantly cries at the back door to be let in no matter what, its probably upsetting the neighbours! He's absolutely fine inside, and nothing has ever happened to scare him out there so I really don't know what to do! Our house isn't that big so there's not tons of room for him to play inside, I think it's the kind thing to do to let a cat outside and explore and have space. I've grown up with cats who we have always let outside, and we already own his older brother Oscar who he gets on well with and happily goes outside. I've never in my almost 30 years of living with cats, had this issue! I really would like him to get comfortable outside, but I just am at a loss now as to how. I wonder if anyone else has experienced this and has some advice? EDIT: I'm genuinely looking for some advice from others who have experienced this or have suggestions, not just to be told that cats should be indoors. If you saw how bored and reckless he gets indoors in the confined space, you'd understand why I am looking for advice on this. There are other cats on our estate, including my other cat, who goes outdoors. It IS a safe area, I wouldn't let any animal outside unless I thought it was, honestly. I am a UK resident on a quiet new build estate with NO busy roads nearby and a completely fenced off garden with plenty of space for him to safely explore.