how long does he nap for duing the day? i think it might be a matter of cutting it back to even half an hour during the day and putting up with a bit of an early rise. you might find that he starts to sleep in a little bit longer after a while.

dont start sleeping with dd and your dh sleeping with ds because it will start a habit that you will find even harder to break. (i speak from experience there) LOL

will he stop crying if she hops into bed with him? will they fall asleep together after having had a little chat and cuddle together?