The answer to “why” Thistle is in one of my earlier responses.
The habits you teach him now form the building blocks for independent behaviour later.
There’s no magic switch at 10 or 11 where you say “all those things I’ve been letting you away with your whole life aren’t acceptable if you are on your own”
You aren’t teaching your son to be a sensible two year old, you are teaching him to be sensible 11yo/14yo/18yo.
It takes a long time. It starts now.
For example my D.C. both have lovely manners. Other parents always comment on it after play dates, parties or sleepovers.
They don’t do anything especially miraculous but they say please and thank you, they compliment the meal, they ask to be excused from the table, they offer to help clear the table, say “thank you for having me” etc”
Other parents regularly say, “I wish mine did that” but good manners aren’t inherent, they are taught.
And they aren’t taught overnight it’s years of practice and reminders and praise. Years. And it started at about your son’s age.
It’s easy to teach but you have to be consistent.
Risk assessment/safe and considerate behaviour is exactly the same. It’s not rocket science but you have to teach it, discuss it and you have to give reminders so that one day many years in the future your DS makes good independent choices.