Hi Becky!
Firstly, and you may have already realized this from the answers, you're not alone. Every parent has told me that their child has done this when switched into a bed, so don't worry.
We went through something similar with our daughter recently when she was 32 months. We switched her from her crib into a double bed when we moved into our new home. For the first month, she did great. Then she started to wake up in the middle of the night--first once a week, then twice, then it started to become every night.
I would always take her back to her bed, lie down with her until she fell asleep, but I knew I couldn't keep doing this--so I wrote into Mamapedia and asked my friends for advice. In the end, b/c she was older, too, I told her that she had to stay in her room and if she could not, we might have to put a gate on her door.
I also put a nightlight in her room, which I think was the answer. Ever since putting that in and having that conversation, she has stayed in her bed. I also rewarded every day for that first week that she stayed in her room, so she knew she was doing something great.
If you want him to switch to a bed, then I would not return him to the crib--I think that would send the wrong message maybe? But as others have mentioned, I'd work on one thing--either the potty training or the bed--not both at the same time.
The key is to be gentle, but firm. One other option is to put a mattress in your room for him to sleep on, but that is also opening up another can of worms :)
Good luck and hope you find something that works soon. There are tons of answers on Mamapedia related to this as well if you have time to search through them.
Take care!