Uhh I dunno haha we just kind of told ours.
First time round, Lydia was 22 months when her first brother was born. When my belly was getting noticeable, we started talking about how mummy has got a baby in her tummy and it is going to be a little brother for her, and that he was going to be called Alex.
Second time round, when we found out I was pregnant, Lydia was 3 and a quarter, and Alex was 16 months old. I took the pregnancy test and told DH, and he basically just shouted over at them "KIDS, YOU'RE GOING TO HAVE A NEW BROTHER OR SISTER!"
Lmao
Alex didn't bat an eyelid, and Lydia was like "Oh...okay" haha.
Third time round, when we found out I was having Nathan, Lydia was 7, Alex was 5, and Jason was 3.
We sat around the dinner table having takeaway pizza, and we just said we had something important to tell them... and we gently told them that they were going to have a new brother or sister because there was a baby in mummy's tummy, and that it was a huge surprise even to us. They were all really excited.
Just tell your son, it's not as big a deal as you're building it up to be. He'll probably take it for granted whilst being excited at the same time.