K.N.
I remember reading an article that said that children typically do not develop the mental milestone of understanding imaginary boundaries which are not actually confined ("stay in bed") until closer to age 3-3.5. Hence, if your son has not yet developed an understanding that the edge of the mattress is the boundary line, then you may be setting him/yourself up for failure until he does hit that milestone. Can you put him back in his crib for several more months and then re-introduce the toddler bed?