My ds is 3 and I'm 15 weeks and we have the book linked to earlier, which is ace for him, but then he is that bit older. We've also seen lots of friends who 'had a baby in their tummy' and then we see the baby once born so he understands that much.
We've also had to educate him on how delicate mummy's tummy is 
What I don't think I can prepare him for is what an actual baby is like. Or not without putting him off the idea
. It's hard because people say things to him like 'will you play with the baby' etc but really newborn babies are noisy and dull for a toddler!
We were talking about toys earlier though and he was asking what toys he could share so I was trying to show him the sort of baby toys that you have but it's hard for him to understand.
To be honest, it's hard to prepare him for life with a sibling since i have no idea really what it will be like!
Just hoping we can all adjust when the time comes.
Apologies OP, that doesn't really answer your qn as your ds is so much younger, but I would say you'd probably struggle to get them to understand too much about what's to come. Even if they 'get' the pg, that's a whole diff ballgame to life with a baby.
I wonder if there are any good books for that bit?