Show him the ultrasound scan and tell him that you have a new baby in your tummy. I believe it's good to get the older sibling involved right from the start to minimise chances of sibling rivalry. Get your child to accompany you to ultrasound scans and checkups and let him choose outfits for his sibling. Some children can sense pregnancy even before being told. Children are extremely sensitive, and they will notice even little changes about their mother. http://sg.theasianparent.com/6-tips-on-telling-your-child-you-are-pregnant-how-the-experts-do-it/ http://www.deseretnews.com/article/700043670/Child-Sense-Pregnancy-and-the-senses.html?pg=all
From my previous experience, i brought my no.1 with me for the ultrasound scan and tell her there is a little baby in my tummy and you are going to be a big sister. Also, i borrowed sibling books from library for her to understand more about younger siblings and what to expect when baby is born.
thanks