Tan, it must be hard seeing this in your son knowing how it has affected your own life. I am not sure if there is much I can say to help but there are a couple of things that come to mind.
First my DD was also not breathing when she was born, and taken away for 20 mins while they worked to get her breathing. After a quick kiss and photo on my chest she was whisked away to NICU where she spent the next 40 or so hours. She is far from anxious, a robust, independent, cheeky girl so I seriously doubt that what happened immediately after L's birth caused him to be this way.
As far as whether you have made him this way through your parenting, I know a member's DD who has been parented in a very similar way to L - cosleeping, almost always in a sling, BF on demand etc and again, she is not anxious, she has a sense of inner security you can see it in her face - she looks like the cat that got the cream . So I really don't think it is your parenting. I agree with PPs that if you had dealt with him differently if anything his anxiety might if anything be worse.
As for L and A being so different I think they are just inherently very different personalities.
I agree with the PPs - have him properly assessed as it sounds as though he will need support to begin school next year, and hopefully your GP or Paed can point you in the right direction.
You've given it a huge amount of thought and will have heaps to tell your doctor to explain exactly what the problem is, so I hope they can find the right support to help you both.