T.F.
hi J.,
It is really a matter of opinion with a toddler bed. When I was pregnant for my second son, we wanted to get my oldest son out of the crib so he would forget about it before the baby came, so he would not feel the baby was taking "his bed"
He was 18 months when we transitioned him and it went great. I just recently transitioned my 2nd son into a toddler bed at 20 months...he also did awesome.
Some people believe keep them in a crib until they show signs of climbing out. But if you plan to use the same crib for the new baby...I would take him out atleast a couple months prior to the new little one arriving.
Good luck
T.