DD1 did this for ages when first going into a beg bed. (We put her in a big bed at 15months). I think some of it was the novelty that she could get in and out of the bed. Allot of it is perserverance. Doing/saying the same thing everytime we went in there (it took a bit to get DH to do the same as I do). Also having a regualr routine before bedtime... here it's lunch, 1 ep of Hi 5 then bed at night it's dinner, bath, books (or a wiggles show depending of what DH and I have on) then bed. It took ages but it seems to be working. And the same as your DS, DD would go to bed no problems everywhere else.

I know this probably isn't helping you much or what you wanted to hear. There isn't any quick fix solution (or not that i've found anyway...)
GL babe. Hope it settles for you soon. xxx