Hi there
Im sorry about the late reply but been away from these forums for a while, I have two children and there is 16 months between them and i was worried about the jealousy as well.
Keep you son involved in your pregnancy, tell him whats going to happen and how he will be big brother when the baby comes along.
Also when friends come over to see the baby, ask them to make a fuss of your son first and say things "Can you show me your new baby brother/sister?"
Good luck with everything and hope it all goes well
Sian xx