I had both my boys at home (19 mo apart), and the older one had no problems adjusting to the baby, and was always very loving and protecting. (Of course, as they've grown older, he wasn't *always* loving and protecting, like when the baby would mess up his block tower, etc., but you get the idea.) He came in just a few minutes after the birth, and I was holding the baby to my chest, and I told him things like, "Look at the new baby! This is the baby that was in mommy's tummy, and now it's outside. This is your baby brother." And I had him kiss him, which he did without a problem.
Is it possible for your son to be with you when you give birth? Many people have noted that their older children have better adjustments when they're allowed to be there at the birth of the baby, or to come in soon after, rather than being away from mom for hours and then the new interloper, um, baby, is there taking all of her time.
Of course, what happens after the birth is more important, with the daily adjustment your older son will, of necessity, undergo. Try to include him in as much day-to-day things as possible, and explain things to him, even if you don't think he understands. ("Baby is nursing right now - this is how baby eats. I have to change baby's diaper now, just like I change your diapers. See how the baby smiles? He loves you! Hear the baby cry? Babies cry when they're hungry and tired - you used to be a baby, too, and you used to cry a lot.") Read him stories while you're nursing the baby, if you can manage it. Give him one-on-one time while the baby is asleep.
And some days, when both of them are crying, and both need you and only you, and both want to be the only one in your arms... it's okay just to sit on the floor and cry right along with them. It's tough having kids this close together, because the older one is sometimes still a baby, and needs to be babied. But don't worry -- you'll get through it! and then they'll be good playmates.