News + Baby Post-Vacation Sleep Help?

Yesterday, we had our big anatomy ultrasound.  Baby cooperated nicely (despite being super wiggly) and let us know that it's a GIRL!

I'm still a little in shock - I can't quite believe it.  I think Dan and I both expected it to be another boy, but we're super excited for her and for those two little ones to teach and love one another.  Yay!

On another subject, I need help.  We got back from a 2.5-week trip to New Zealand and Australia late Monday night, and Nat's sleep has been, frankly, miserable since.  Well, it's not even really the sleep.  It's the refusal to be by himself.

You see, we were spoiled.  I'm not exaggerating when I say that he was sleeping 8 hours at night by a month old, and 12+ by 2 months.  Ever since dropping his morning nap around 13 months, he's slept, with just a couple of little hiccups, 11-12 hours at night and 2-3 hours during the day.  For his naps, I would turn on his sound machine, hold him for a minute or two while singing one song, and put him down in his crib completely awake.  He'd drift off to sleep all on his own.  In the rare instance when he really wasn't tired (or after he awoke from his nap), he'd play happily in his crib, chatting or singing to himself.  In addition to this, he'd regularly play for upwards of an hour by himself, just reading books or playing with toys or doing whatever.

I fully acknowledge that we got totally lucky and that this was through NO amazing parenting feat.  I'm even more convinced of that now as all of that has gone down the tubes the last few days.  Now, when we're with him, he's still perfectly happy and giggly and playful.  But during the day, he insists on being held for much of the time.  He falls asleep fine when we're with him, but wakes up when we leave the room, or freaks out and screams bloody murder if we leave him awake in his crib.  I put him down about 45 minutes ago (when he was clearly really tired) and he's been crying since (I went in once to comfort him and explain that I would be in the living room and he could come out after he'd had his nap).

I'm fairly certain this has to do with some sort of post-vacation separation anxiety.  He was with both of us pretty much 24/7 for nearly 3 weeks, and slept in the same room as us.  If he woke up at night, we immediately picked him up and brought him to bed with us for fear of waking up any hosts or neighbors.  And now, he's clearly used to that and is terrified of being by himself in his room.

So, HELP!  All you experienced moms, please tell me what to do!  Do I just let him cry for forever and hope he figures it out?  Do I rock him to sleep for a few more days to let him adjust?  Do I just pray like crazy that he remembers his good sleeping habits (I think that one's a definite)?