I've looked after lots of babies before and after cardiac surgery. It's bloody hard for mums and dads but you will be well looked after, be allowed to spend as much time as you want with your baby and although you won't be able to cuddle him straight away you'll be able to hold his hand, stroke his head, help to wash him, change his nappy and so on.
If you can express milk I'd really encourage you to do this. Babies tummies are really delicate after any sort of surgery so your milk is the very best thing for him and as well as helping his recovery you'll feel you're doing a 'normal' mum and baby bonding thing.
Another thing i would suggest is that you get some little muslins now and sleep with some of them under your pillow or tucked in your bra so they smell of you. Then he can have one that smells of you, and a clean one, to take with him once he is born. Then when you're expressing for him, you take one that's been snuggled up with him so you've got something that's been close to him.
It's definitely worth going to see the unit where he will be, they're not as scary as you might think, you'll see poorly babies who've just had surgery but also the babies who are getting better, off their ventilators and getting out of bed for cuddles, you'll meet the nurses and see just how closely he will be looked after and how much they care about the babies and their parents, so when you actually come to see him you can just concentrate on him and not be shocked or confused by what's going on in the rest of the unit.
Good luck, it is a scary time but yoi will be looked after xxx