Is his 3 year old sister kind to him you say they shared a room,when you were on vacation, He also slept many hours for a 2.5 year old 12 hours then another two during the day , try cutting some of it back,he may not be tired enough. You have to be consistent as he is also getting mixed messages, he knows if he yells enough you will go into him, best to sit in the room and leave him there .or leave him to yell.,once you give way you are back to square one. .
Our son was an awesome sleeper... much better than his sister about going to bed and sleeping through the night. At his 3rd birthday our son entered preschool for the first time, had strep for 3 months, would wake up screaming from sore throats, plus we were totally stressed due to the economy... and it was like the floodgates burst. For 6 months he was up 4-5 times a night, and started all kinds of anxious behavior. We had to do something... we started sleeping with him, did everything we could to reduce his anxiety... took him out of preschool, changed our routine to be more consistent, changed our discipline techniques, limited his TV before bedtime, no movie theaters (he had only been once), reduced our own anxiety through several means... It has gotten better. About 10 months after it all started - he finally slept through the night!!!! We still lay down with him to get him to sleep, but he drifts off faster now and doesn't seem to mind we sneak out. We will start trying to get him to fall asleep without us as the next step...
We have all kinds of other issues to work out with him, as we've discovered he has Sensory Processing Disorder which has fed into his anxiety, as well as potentially Post Traumatic Syndrome from an event at preschool... but at least the sleeping has gotten much much better...
SO I hope that you are able to find ways that works with your families to help lessen the anxiety.
We moved about a month and a half ago and we are having this same problem. My 2 year old would go to bed without crying every night and sleep 12 hours, prior to the move. Now she screams and cries every night and every nap. Then she was waking up during the night screaming and if I didn't go get her she would stop crying and just stand in her bed...sometimes this was from 3am on. It has progressively gotten worse and worse. I asked the pediatrician what to do and she said to go in, pat her back, sing, etc to calm her back down to sleep. Yeah, that didn't work!!
So now we have let her back in our bed at some point during the night, but no one is getting rest. I'm sure she was overtired & overwhelmed with our move because I sure was.
I have a friend who has had luck with a 'sleep fairy' leaving little treats for a good night of sleep, others who swear by not going into the room no matter what, etc.
I'm so stressed out trying to decide what the best answer is! I think we are going to switch her back to the crib & start over with crying to sleep. I feel so bad because it seems like she needs me!!
Does he get a nap in the day? You could try withholding that but that is pretty drastic- or try putting him to bed 30 mins later every night for a week then move on to 8- 8:30- 9:00 til he gets back to sleeping toward daylight?