K.L.
Being potty trained and stuff sounds like he needs to be in a bed so he can get out if he needs to, plus if he's ready for underwear, I bet he's ready for a toddler bed! ....but kids can be funny about things. My 2 year old started crying fits a few months after we put him in a bed..and we moved the position of the bed in the room and it stopped. With each of my kids, we went out to the store and picked out a special set of sheets or something to make them feel special..then included them in the process of washing them, making the bed (not right before bed though), and it made them excited to be able to sleep in it!
My biggest suggestion is not to go back and forth! If you make a decision to put him in the bed, don't even dicuss the crib again...you'll loose credibility to him and next time he's just going to cry harder and think that he'll get his way! When you put him to bed, have a ritual that you go through, remind him you're proud of how big he's getting, tell him you love and and that you'll see him int he morning and goodnight! Good luck!! :)