When to transition to a toddler bed

Jackie • Nurse for home health. mom of 2 rainbow babies
We are expecting another boy in about 12 weeks.. we have an 18 month old who is still in a crib. We dont want to buy another crib so when is a good time to transition? He has been sleeping in bed w me for the past few days as he doesnt like his crib anymore but I cant get a good sleep because of this. We have a video monitor and his room is free of any toys (thank god for a playroom) and all safety measures are taken.