Hi Tracey,
You can just wait until you are starting to show and then let him know he is gonna have a new baby brother or sister or wait till you know that if you are gonna find out.
No matter what the age it's always best to follow theses 2 rules ...
1) be honest
2) put it in age appropriate terms for the age of the child.
You can also get him a doll and show him how to gentle touch and how to hold the baby on his lap and he can practice feeding the "baby".
Oh and for you, when he asks for something to drink or wants you to get him something stall for five to ten minutes before you get it for him. This will get him used to him waiting patiently even if you don't have something to do right now.
Get the babies things out a few months before baby is here so he is used to seeing the new things around like swings and things like this. Get him used to the idea now not to hang on them or play with them.