My son is 21 months old, and there's just no way on earth he'd get the concept of being a 'big brother'. Your son's whole world is about to be rocked, for the better of course, but still...he's no longer going to be the baby or the focus of all your attention. I say leave his nights as they are now...total upheaval day and night will not lead to a happy almost-2 year old. In a few months when he's adjusted to life with a baby in the house, *then* start transitioning him to a toddler bed in your room, and then to his own bed in his own room.
Our oldest was 3 when our second was born and he was cosleeping. Rather than pull the rug out from under him, we took one side off the crib and turned it into a sidecar and that's where the baby slept. After a few months, he moved to the toddler bed and the baby moved into the bed. He let us know when he was ready for his own room a few months later and transitioned painlessly and peacefully. We did the same with our daughter, and she was in her own bed by the time she turned 3.
I get so tired of all the "oh my goodness, how could you allow such a terrible thing to happen?" posts. Cosleeping can be a wonderful thing, and I know that it's one of the aspects of babyhood and toddlerhood that I'll miss the most. We had no sleepless nights, no nightmares, no covering our ears and hardening our hearts against the cries of our babies.
So, in short, I say buy a cosleeper and leave him where he is for right now. Put yourself in his little shoes and ask yourself what he's going to need from you in order to make this transition that's ahead of him.
M.