First off, congrats on your pregnancy! i hope it goes smoothly and healthy.
My advice would be to switch him to the new room as soon as possible (if you have it ready by now, better since he'll get a chance to spend a full 6 months in his new room without blaming the new sibling) even it this means the whole arming/disarming tango.
I'd say your ds is still very young to switch to a toddler's bed but some moms around here have found success at that age, i guess it all depends on how sensible your toddler is to changes. If he's a spirited/touchy/grumpy one, i'd suggest you to take baby steps (that is first, move him from the room, then a couple of months later to the bed and then leave a good 2 month period until baby arrives), but it all depends on your timings. Some moms have had to do the switch all at once and even if it was not a pleasant process, it lasted shortly and the lo's got used to their new surroundings quite easily.
HTH and good luck!