I may not be the best person to give you advise on this issue, because both my 4yo and my 2yo sleep with me (my husband works nights). It all started because my 4yo was super clingy, and never would sleep in her crib. The 2yo slept fine on his own till we went on vacation, and he learned to climb out of his crib :). I'm not suggesting you let him sleep with you, that would be even harder to break. However, I wonder if putting the 2yo in bed with him would help him fall asleep. Is it simply that he wants another body there, or is it more that he just wants dad or you there. For my 4yo, it's that she just wants me all the time. My oldest (12 now) was never clingy and was very much the oposite. Always slept fine, and actually never really even gave hugs or kisses, even when I begged when she was little :). So, I probably let my 4 yo get away with sleeping with me because she wanted to be close. Although, she used to throw up every time I put her in her crib when she was little, which was the other reason it all started with her.
Any how try to see if the 2yo can sleep with the older one. See if that might work.