I had an angelic little 1 yr old. We read books about the baby, he knew there was a baby inside, he was excited... then his little sister arrived and he turned into a BRAT! But I realised it wasn't true misbehaviour, he was just trying to deal with not being the centre of attention... so I tried really hard to give him all the extra attention I could. He was still pretty bad for 2 months, very jealous and lots of tantrums. We went out a lot as a distraction. But it slowly faded away and he adjusted and now he is 2 and she is 1 and they play together so well! So you never know how it will turn out I guess but I'm really glad for the small age gap now, even though it was hard at the start... and you might be lucky and he might love the baby... his own doll or 'baby' to take care of might help (it didn't with my boy - he wasn't interested... only into cars at that time)
Once you start showing talk to him about it. My first two are 17 months apart. I would run my belly and tell my oldest child there was a baby in my belly. Once I found our the gender I would tell her it was her baby brother. I showed her how to gently rib my belly. And when we would be out seeing other baby's around I would say Mommy is having one soon. Also you can find books to read to your son about becoming a big sibling. Yes he's young, but they understand a lot more than we realize.