I agree, having a child for the sake of a sibling for the child you have is silly.
There are a lot of pros and cons to either situation - i guess you have to discuss this with your DH.
My MIL was an only child, only had her mother (father died at war) and they had no family around. YOu can tell she is a only child. Very 'me, me, me' not in a spoilt way, but in way where i guess thats all shes had to contend with when she was younger - herself and her mum. She often admits how lonely and boring her childhood was. But this was 60 odd years ago. Society and lifestyles have changed.

Travel, education, relationships, opportunites are endless for you, your partner and your DS should you choose to only have one child.

I know a family with just one son. - he is 7 years ol. He and they are 'normal' family, going to sports of a saturday morning, taking him to the rugby league match of an afternoon. They are great parents and in no way is there son spoilt. Infact he's very confident and genuinely a nice little boy.

Good luck with your decision......