DD1 was only 15mths old when i had DD but we constantly spoke to her about the baby, that a baby was coming to live with us etc etc, i don't think she got it but i have photo's from when they first met and there was this maazing instant bond.
DD1 looked so proud (odd i know) and content with her little sister.

I think just keep explaining to him, reassure him all will be good, maybe get him a gift from his little sister and get him to pick something for the baby.