Hi CK!
Congrats on being pregos again! How exciting... or will be more so when you realize this is a great gift from God for your son and the rest of the family!
I have 17 YEARS between my children and still felt the same way as you are feeling now. I worried how could I love another child as much as I love my son. I worried how would I make my son, even though he was an adult almost, feel like he was still special and just as important as the new baby coming. This is all normal feelings you are having!
With that being said... Your son will still be the center of attention (after the first week or two) because he will make sure he is. He will want to help out, get diapers, help feed, put things in the garbage etc. You will include him in to help him bond with his new sister or brother and give him constant praise for his efforts. That is postive attention he needs as he is growing anyhow but now he gets to be the Big Brother!!!
You are worrying about him being able to have Mom and Dad time... make it happen! Even when my son was an only child we had "date" nights with him. Even just a trip to the grocery store was time alone together. Plus there will be plenty of time where the new baby will be sleeping where you can pull out the story books and just snuggle on the couch reading or tickling or being silly as well as a million other things you can do!
I personally think your son is very lucky, first to have such a loving Mom who is concerned about how he will react, but even more that he will have someone to play with and have as his best friend. You have been truly blessed to get pregnant again so close!
I wish you and your family many blessings!