A.A.
My first daughter was only 4 months old when I found out I was pregnant again! It was challenging in the beginning, but my girls are the BEST of friends. They always have a playmate they can relate to no matter where they go. It really turned out to be the best situation for us. My eldest daughter loved to have a LOT of attention when she was younger and since I was a single mom I couldn't always meet her attention quota. Lucky for she and I, her adoring little sister gave her plenty, so she was satisfied. Even though it was certainly not planned that way, I can't imagine it being any different.
I have always tried my best to give each girl some individual mommy time in the way they each prefer to connect with me. My eldest likes to talk and my youngest likes to snuggle... As far as your son losing time to be an only child, your second child will never be an only child, so I wouldn't worry about it. He'll adjust.
Congratulations on becoming pregnant without having to use fertility treatments!