I would say it really depends on your son. Is he totally in love with his crib, or is he always trying to find a way out of it? If you bring up the idea of a Big Boy Bed does he even like the idea?
My daughter was a few months shy of 2 when my other daughter was born. I was freaking about getting her into a bed before we brought the baby home so she was used to it too. I think we did it a few months before. We made kind of a big deal about it and made sure she knew it was her BIG girl bed. We did the special sheet thing too and asked which friends she wanted in with her. She went over pretty easy and understood the crib was a baby bed. There were a few nights of getting up, but nothing too bad. We have it set up so if she wakes up at night, she can come right into our room.
I think my advice would be try out the idea with him, then try out the bed. If he totally freaks on you, then have the baby in a bassinet next to your bed for the first few months. Most go up to about 15 or so lbs. We always start out with ours in the bassinet anyway.
A bedtime hint if he has a hard time getting in there- my daughter has a bff stuffed penguin named Buddy and we tell her he is tired and she needs to tuck him in. It works like a charm! We think it's because she gets to be the boss of someone!
Good luck and congrats on your family!