Very nice drawings and atmosphere in this

It's a neat idea- I'm glad to see some variance in shot-types and that you use sound effects to tell the story as well. These can definitely be pushed more.

I don't really feel satisfied at the end, it seems just as it's getting very interesting, it stops short. Who is the rat? What does it do with him? Is it actually in his mind? You don't have to directly answer all of these (though some direct answers might make this story feel more resolved), but give some clues for the viewer.

With practice the animation will become less stiff- timing makes all the difference. Think of layering sounds together, and what kind of effect that has on us as an audience, instead of simply using them when the script calls for one. All rooms have ambient noise, even if it's just a hum- you could definitely take advantage of the ventilation duct this way (something like this could help a lot in those long scenes in the dark.)

There's a lot of room for growth here, good luck on your next project!