I have two children, too. My daughter turned 5 in Jan and my son will be three in Aug.
I moved my daughter to a twin bed (toddler beds are a bit of a waste). I did it 4 months before her little brother was born.
When you move him, move the crib out of the room (if you can) and put it in the baby's new room. make a big idea of the fact that your little sweetie is a big boy and that the bed is now for a baby. Maybe put Thomas the Train or Cars sheets on his new bed-whatever he likes. Put the bed next to the wall and put a bed guard (about $20 at a baby store-or less at Once on a Child) The guard will keep him from rolling out. If you don't want to buy a bed guard, make sure you put pillows on the floor around the bed for the first week or two.
My daughter fell out the first two nights--then after that she was okay. The guard just helps them stay safe. I removed the guard when she was ready to night-time potty on her own (around 4).
There may be some nights when they get up--just take them back to bed, and lay with them a little while to make them feel secure if they need it.
we've never been co-sleepers, so, she always understood that was her bed and we had our bed. I am blessed because both kids sleep well. There's an occasional night here and there that one of them wakes up, but I feel very fortunate.
My little guy will be 3 in August, too. I don't have a little one to put in the bed--and he sleeps great in the bed.
As for me, with transitioning, #2 out of the crib, I am waiting longer just because I don't have another babe to put in the nest...SIGH. I will probably wait until Nov/Dec as I will need to buy him a bedroom set. But who knows, that could all change tomorrow.
Bottom line, trust your instincts, and do what feels best for your little guy.