I just had our second son and my first son was 15 months old. We started 4 months out with an inexpensive (and age appropriate) doll from Wal-Mart - something small enough for him to hold. We reacted positively and with excitement about the doll as we showed him how to "love the baby" (hugging and holding), how to "be gentle" while touching the baby and how to "give kisses" to the baby. He also learned to tap my belly when we asked him where the baby was - he seemed to understand on some level. With your son being a little older, he will probably learn more quickly and can be taught how to help wash the baby, change the baby, etc.
This not only prepared my son for the physical presence of the new baby, but also some of the activities he'd see, including seeing mom and dad holding the new baby all the time! We have always referred to our second son as "your baby" to give him a sense of ownership.
Our second baby is now a month old and my first son is enamored with him! Right after he was born, my 15 month old immediately leaned in to give him a kiss on the head. Now the first thing he asks when he wakes up is "baby?" and he makes the ASL sign for baby. I tell him where his baby is and tell him about/involve him in our activities - nursing, dressing, etc. He's allowed to choose which outfit to put on and we snuggle and read a book when the baby nurses. He wants to give his brother kisses all the time and he's very concerned about him when he cries - so much so that he'll come tell me "baby" if he hears the baby crying from his bassinet, or he tries to put the baby's pacifier back in his mouth if it's fallen out and the baby is within reach.
At your son's age, you might even look for a book like "I'm a Big Brother" or "We Have a New Baby" to start reading to him to prepare him. You might also want to have him help pick out a toy to give his baby brother when he's born.
I believe the more you involve your older son, and the more ownership he feels about the new baby, the more excited and accepting he'll be.
Best wishes and congratulations to you!