At the end of Room, Jack and Ma are on the bed playing Lego. He tugs on her sweater and says "Can I...". She cuts him off and says "there's no room left".
What does this mean exactly?
At first I too did not understand what he wanted. But if you remember, when they were trapped in the room during first-half of the movie, Joy used to breast feed him a lot may be because of the lack of food.
So, toward the ending of the movie, tugging at her t-shirt, he asks for her milk but she says she doesn't have left any.
There's no room left.
The exact conversation is,
Jack: Can I... (tugging at her shirt)
Joy: No. There's no more left, sorry.
I think, most definitely, Jack's asking her to breast feed.