I'm an only and so are both my parents - so a lot of onliness here!
I have 4 DDs, but no.4 is ALOT younger than her 3 sisters (who are v close in age) so is like an only.
Having had 3 DCs close in age and 1 like an only, and me being an only, i feel i have a good perspective on it all.
Total honesty:
I had a happy childhood, lots of friends but played alone for hours happily and I learned reliance on myself and as an adult i am good at being alone without company in the house. (DH isn't good at this, he's the youngest of 4)
However - now my mother is in her 80s i wish i had someone to share the burden with. Also, i see my older 3 (in their 20s now) reminiscing about their happy childhood and i wish i had that life long connection with someone else.
My DD number 4 has a personality not very suited to playing alone, as mine is, and she talks about having a younger sibling often and pesters for me and DH to play with her a lot.
One pro is that we've been able to afford more for DD4 than we did for her siblings as paying for 3 to do something is so different than 1.
My older 3 girls are still very close, but DH hardly speaks to his siblings (all in their 40s now) but they are there for each other if needs be, y'know? I don't have that.
But it is what it is! You've given your son life, OP. And he'll be happy.
At the end of the day, I don't know anyone who would say they'd rather not exist than be an only!