I have a DB 3 years younger than me. We were quite close when we were little, less so when we were 12/9 onwards and grew to be very different teens.
We have always lived in different countries since I was 18 (30 years!), so see each other maybe once or 2x a year, so not so close - but when we see each other, there’s a real connection, and we both know we would do anything for the other one.
My own kids are DS, DD1, DD2. 15, 13 and nearly 7. The first two were very close until they started school, and then got on- but not as close- friends were closer.
Then when they were 8 and 6, I had DD2. DD1 adored her, and they were a real twosome, and poor ds seemed a bit of a 3rd wheel - but also wasn’t that bother about a baby/ toddler.
But as soon as the older ones were both at secondary school, and travelled by train on the same route, this shared experience made them closer, and the age gap between dd2 and the older ones seems bigger.
Dd1 at 13 finds a little vastly irritating, whilst DS at 15 is suddenly much more sociable and goes bouncing on the trampoline with DD2 whenever she asks, gives her piggy backs on days out, and suddenly has a lovely relationship with her.
I have found watching the wax and wane of these relationships fascinating, and hope they choose to be close as adults, but realise that time, circumstance and geography play a big part in all of this too - as much as gender.
Congratulations on your little boy !