With my first Ds he moved into a big boy bed when was 2 years old to make room his baby brother.

it took some getting used to but just kept reminding him its his bed and his only and he is such a big boy now and made a really huge thing out of it. like going shopping for sheets and doona covers.

With my 2nd DS i dont think he will have any problems he loves sleeping in beds espeically his brothers sooo its not such a bad transition

just gotta stick it out