That is exactly what happened to me almost 10 years ago!
My oldest boy just turned 4 when I met my husband. I was also very close with my son and I said the samethings you said, I even said that I was never getting married again!
We met in Sept. and in Dec., I found out I was pregnant with my daughter.
We got married in March and had our daughter in Aug.
Through the entire event of our being together, the wedding and the preparations through the birth of his sister, we involved him in as much of the planning and prep that we could.
Before the wedding, we would go out and do "family" stuff together and my husband paid alot of attention to him too.
He was a huge part in the wedding, he stood up as a "mini" best man with the actual best man.
When we were getting ready for his sister, we bought a special doll and we treated that doll as a real baby. He learned how to change diapers (as well as a 4 year old could ;) ) and when we would go out, we had the carseat in the car and put the "baby in it and had a doll sized stroller so he could push the "baby" around in the store. We didn't do this every day, but we did it more ofter when it was getting closer to her due date.
It really did work for us, he loves his step-dad and he accepted his sister right away and was a big momma's helper when taking care of his new baby sister.
My husband and I will be having out 10 year wedding anniversary in March this year. We met and was married within 6 months.
We now have 3 children total. We had a boy 7 years ago.
There ages are 14, 9 and 7 and all is going well.
Good luck in whatever you work out with your son, go with your instincts and I'm sure he will do fine and really do well with having a "new" daddy, mine did :)
Congratulations on the new baby and up coming wedding.