You have some great advice already! It's good to be aware that this can happen but try not to let your own fears burden you so much. The great thing about being a parent, is that you have a very strong influence, and you're there from the start. Before you even have the baby, you can buy a book and read it with your older son. We had "I'm a Big Brother Now" http://www.tourvacationstogo.com/adventures_by_disney.cfm...
It teaches the older one that there will be a new family member, but emphasizes that he is still important and plays an important role. Mine loved to "help" with things when the baby was born, even simple things, like hold the bottle while I burp the baby, or grab me a diaper, or go get the wipes, etc. It builds self-esteem and shows him how being the big kid has its advantages. He seems so young to you, but you'll be surprised at how many things he really can help with. After he does, give him lots of praise, "What a great job! (Insert baby name here) is so lucky to have such a great brother to help take care of him! And Mommy and Daddy are lucky too to have such a great helper!" I also tell my sons frequently, "This is your best friend forever." In fact, before bed every night, they both throw their arms up in the air and yell, "Brothers! Brothers! Brothers!" You'll see, it can really be a great thing. Someone to always play with in the house (good for Mom too!)Even though they won't always get along, if you tell them often enough that they're best friends forever and it's their jobs to take care of each other, they will believe it!
Lastly, make sure to carve out some one-on-one time with him. I used to even read books to him sometimes when the baby nursed. When the baby napped, we would color, and on the weekends, he would help Dad wash the car....things like that.
Anyway, lots of luck to you! If you believe they can be best buds, they'll believe it too.