AnnieYu,
My case is same as yours. Previously, my son slept on his own bed and share the room with the maid. During the last winter, he always got sick and cough very seriously. So he started sleeping with me. Now, whenever i said he has to sleep in his own room, he will cried very seriously. At the same time i also worry him did not have blanket during nite time and get sick easily.
My son is now 3 years old and still needs to wear diaper during nite time. I plan to let him sleep in his room this summer and train him not to use diaper. I think at the beginning i will sleep with him together on his bed. Though it is more crowd, i hope to make it a habbit for him to sleep on his bed. Later on, will train him to sleep alone. Step by Step.
Anyways, I enjoy sleeping with him as it's sooo sweet for both of us.