Hi H.,
The only quick and easy solution is to put him back in the crib. I know that is easier said than done (planning to use the crib for baby number 2?) but that really is the simplest solution to the problem. I will share with you my experience with two kids, as well as something I’ve learned recently on this subject, and hopefully it will help.
When my older son was 18 months, we tried to transition him to a “big boy” bed. Up until then he had been a great sleeper. We had a heck of a time getting him to stay in bed, and now to this day at 3yo, he still has certain sleep issues. Looking back, I believe the bed transition was the beginning of his sleep problems and wish we had kept him in the crib longer. With baby number two, we tried a toddler bed about a month ago (at 20 months), and he slept great during the night (also a great sleeper after being sleep trained as an infant) but got up at 5:30 every morning! No matter how many times I would put him back in bed, he would pop right back up again and would not go back to sleep. After some research on this, I found out that until they are closer to age three, toddlers really do not understand the concept of staying bed, as they lack the impulse control at that age. Likely what happens is, in the crib, they have a brief period of wakefulness in the early a.m., but knowing they are confined, roll over and go back to sleep until their normal sleep cycle is complete (7:30 for my son). However, in the bed during that brief period of wakefulness, they get up because they know they can, and they lack the impulse control that tells them to stay put and go back to sleep. They know they can get up, so they do. Once they are about age three, there are a lot of techniques that can help, (like a clock with the minute hand removed, letting them know that can't get up until the hour hand points to the seven). Seems like 2yo is just a little to young for that.
After one week of my son waking at 5:30 every morning (and after finding the above information), I put him right back in the crib, and we have not had a problem since. And yes, the time change did cause a little blip for him, but I would go in and let him know it was too early to get up and he needed to go back to sleep, and eventually he would. After about four days of that, he was back on track. That never would have happened without the crib because he would have gotten up on his own, rather than remaining confined. I plan to keep him in the crib until as close to age three as possible. So far, I am fortunate that he has not climbed out yet, but I do know other babies who have been in the crib until age three and not climbed out, so I am hopeful.
Yes, you can teach him to stay in bed eventually, through various toddler sleep training techniques, but it is a LOT of work, and with my older one proved unsuccessful anyway. I say, especially with baby number two on the way, save yourself the headache and go back to the crib for as long as you can. You really do not need a crib for the new baby for a while anyway if you have a Pack N play. Seriously, she can sleep in there as a newborn with the mattress suspended, and when she is too big for that, you can lower the mattress to the bottom and let her sleep in there for quite some time. I have a friend that actually had her 2yo sleeping in the Pak N Play at night.