Sounds like you are doing all the right things. To be honest, I think there's a limit to how much they can really understand what it will be like to have a new sibling, whatever you say. DS1 was 2.11 when DS2 was born 5 months ago. We didn't do many books - just a couple about how nice it is to have a sibling (my favourite was Annie Rose is My Little Sister by Shirley Hughes).
If I were you, I might ease off on the 'will you help with the baby' bit for now. I can completely understand the thinking behind it, but it might be making your DS feel under pressure (especially if he doesn't really understand what you're asking him to do). I found it more useful to talk about other siblings we know, particularly one of his nursery friends who had recently had a baby sister - I think that helped make it seem more real/normal for him. Also, it might help to talk to him about when he was a baby, and look at pictures - he might be slightly young for that, but most pre-schoolers seem to love it, especially funny stories about what he used to do/say.
Also, someone gave me the very good advice to warn DS that new babies cry a lot, but that it doesn't mean they're sad. Even with that, DS still got a bit upset the first few times the baby cried.
Oh, and if you haven't already, I'd really recommend this book for yourself. I was so worried about preparing DS1, that I kind of forgot to prepare myself. Some people hate this book, as it's quite hard-hitting and does tend to paint the worst case scenario, but I found it incredibly valuable when someone recommended it shortly after DS2 was born (actually, I cried when I read it 
). I reacted in ways I would never have imagined when DS2 was born (largely to do with falling 'out of love' with DS1 for a while - the opposite of what I had expected), and if I'd read this book first, I'd have been a bit more prepared.
Good luck, and enjoy - having two is wonderful, in spite of the tricky bits!