First thing I'd say is stop feeling guilty about this! Your son will still be spending most of his week with you, and you'll probably find (like I did) that you need a bit of time for just you and the baby (otherwise the poor baby gets almost totally ignored and you start feeling terrible about that - I've been there!). The timing obviously isn't ideal, but you're doing the best you can.
Saying goodbye and 'see you later' is really important, however distressing you may find it initially, so that he knows exactly what is going on - I'm sure it's far worse for them if they turn around and suddenly discover you've sneaked off, it's bound to make them more clingy. I found with my son that he was sometimes upset and clingy when I left, but other mums told me that when they came to drop off their children he was playing happily.
Remember that, if it is a good nursery, they will have experience of children just like your son. They will make sure that he gets his fair share of the toys, and also that he gets plenty of cuddles if he needs them. The staff at my son's nursery have always been brilliant with him (both when he needs reassurance and when he needs squashing!). They also understand that mums sometimes need a bit of reassurance! (I'm sure there were days when I sat outside in the car feeling awful while my son was already playing happily inside!).
Another important (but hard) thing from my experience is that, when you come to drop him off and he is upset, you must try and be really calm and bright so that he doesn't think 'oh, mum is rattled, so there really must be something to worry about' (hence the sitting in the carpark feeling fragile after an Oscar-worthy performance of happiness and jollity).
Sorry for rambling a bit - it's a hard subject. I hope it all goes better than you fear!