Truthfully? I think I'd wait a bit longer. Since you're not even pregnant yet, there's no *real* rush. I know lots of moms who did the transition by just having the BBB in the same room as the crib for 6 months to a year. Especially for children who are resistant to change, this helps a lot.
Then what you do in the meantime is you *really* talk the BBB up! You talk about how it means he's growing up, and all the cool things that big boys get to do -- ride a trike, eat ice cream cones, play with balls -- things that a baby could NEVER do. That really makes an impression on the little toddler mind! They really want to be 'big', so when you talk excitedly about the new big kid things they're going to do, it really helps!
I think if you take some of the pressure off to move into the bed right away, he may surprise you and actually ask for the BBB on his own within a few weeks time. If he doesn't after 3 months or so, then you can make your own deadline in your mind, and when you get within a couple of weeks of that deadline, then you can start really talking it up again.
At this age you can use a Reward Chart to motivate him to stay in the bed as asked. Stickers are great rewards for toddlers, and then I typically encourage moms to offer a BIG prize after 2-3 stickers -- something like a matchbox car or other trinket you get at the Dollar Store.
But if he's quietly crying himself to sleep, that's very sad! He's just not ready for the change. And since there's really no pressure to make the change, I wouldn't push him into it. I'd gently help him to see why it's such a great thing for him! Since his sheets are all staying the same, I think I might go out and buy a special stuffed animal to be his friend in the BBB, or buy something else new and fun -- a cool night light, a special book that only Big Boys can read, etc.Like I said, make him see why it's in his interest to change.
We moved our son at 3.5. We really waited until he was ready and it went super easy. I have a Touchy kid who also has Sensory Processing Disorder, so believe me change is DEFINITELY the enemy over here! My kid goes crazy if we even move something in the living room, LOL! But we just talked up the change, and really stressed all the cool things that a big boy can do. You might also make sure to see if your son is scared of something about the BBB. For instance, my son really was scared of changing and I eventually got it out of him that he was super afraid of a regular twin bed b/c it was so high up to him and he was afraid to fall out. So we started him out in a toddler bed, which is low to the ground. Once he saw how small it was he was excited about the change. See if you can't figure out if there's something else going on with your kid that may be scaring him or worrying him.
Good luck!