B.D.
You can try a couple different things. You can try and get him a fish and he has to take care if it like his baby. Or you can just involve him in everything and anything you do. Have him pick out the baby's clothes, or get diapers etc. Just make sure you are making it fun and not a chore. Also, when your hubby does have a little time off (i know it's rare) take your son out somewhere special for an hour or so, just so you can bond.
As far as when you have the baby, make sure the grand parents keep him the center of attention, not the new baby...which I understand is very difficult. Also, I let my oldest pick out a couple of things for my youngest, as gifts when they were born. That way they felt part of it. I also then got my oldest a new toy or something when I had the baby, so they remembered, that I was still thinking of them.
As far as the nursing and care of the new born, just make your son as much as possible part of the routine. Read your son a book when you nurse. Or play a little board game when you nurse; something that is "alone" time for you and your son, but you are still able to nurse the newborn.
Good luck and Congrats!