Hi Calandra,
My advice is to keep him in his crib until he shows signs that he doesn't want to be there. If he isn't trying to climb out of it and he seems to sleep comfortably in the crib, then wait to transition him. I know you said a few months, but don't keep to a timeline for that kind of thing, just take his cues.
Okay, now once he seems ready for the big boy bed, make a big deal about it letting him pick his new bedding, etc. Our son was 2 when we put him in a toddler bed, he got to pick out all the stuff and was SO excited to try it out. If he gets up in the middle of the night, you can implement a rewards system when he stays put. We used 3 pennies. Everytime our son got up, we took a penny away. He quickly learned that he got to keep the pennies left in the morning and stopped getting up at night.
As far as the crying goes at bedtime, it's not that he should stay up later, in fact he may need to go to bed sooner (I know, crazy...but just try it). The crying could just be from him being over-tired or over-stimulated. Do you have a cd player in his room? You could always turn that on 15 minutes before bed so he can start preparing himself for night night time. You are doing the right thing...don't give up! This is a phase and will pass soon. Good luck!!