JB, CONGRATULATIONS!! You're blessed, you have 9 months to get ready, make a list and relax. Being stressed won't help your baby develop so take a deep breath and count your growing blessings. :)
Everyone feels that way at some point in their pregnancy, it's normal. You will do beautifully and when the dust settles, you'll have a wonderful new baby to snuggle. And your son will love the baby! Toddlers just adore babies. And they'll always have someone to play or fight with, which is great! My kids are so far apart (19, 10 & 23 mos) they had very little in common and I wish they'd been nice & close together. My sis had hers all in a row, at one point she had a 4 yo boy, twin 2 yo girls and a newborn boy. She survived & they all thrived, now they're all taller than she is and are fabulous kids who have common friends and the boys sure keep their sisters safe from jerky guys. :)
Good luck and congrats, again.
D.