Did he *possible spoiler* at the end?

I understood this as a metaphoric image: Even if he escapes through the window, he'll never have the commonplace grounding to fall back to, where society is.

i also think he dies. what stimulates this reasoning is the fact that kyla wanted to leave the neighborhood. she knew the kid wasn't going to be better, and would most likely have a terrible accident, knowing how violent he is.
so he left a final message for his mom, promising her that he'll be better (well better off not there causing her pain and trouble at least).
sadly the phone was on vibrate and she missed the call, which encouraged him even more to take the chance when he got it.

I wondered the exact same thing.

I looked close, it wasn't a door to exit.

Definitely a window, but would they have breakable glass in such an institution?!

He had cut his wrist at least twice before, but he was also real forlorn when they showed him after cutting his wrist in the store, whereas while running towards the window he looked elated.

So I'm not sure his intent/mental status.

Certainly gives that impression.

The way I see it, is that the ending is a reflection of the audience. Whatever you think was the intention of him running towards the window/door, is actually a reflection of your own mindset. Whether you believe it was just another dream sequence, or a suicide attempt, or an attempt at escape, it doesn't matter. There is perhaps one intention that Xavier Dolan had in mind, but I wouldn't want to know it. Mommy was full of suggestions, but we know almost nothing for sure. How did Kyla get her stutter? Why was she on sabbatical? How did Steve's father die? There are lots of suggestions, and we unconsciously try to put together the puzzle all in our own ways. Mommy is partly constructed by Xavier, and partly constructed by our own imagination.