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I'm thinking of writing a story with some sort of shapeshifter race involved, preferably as some sort of animal. (This idea was inspired by the Pokémon Ditto.) The main character meets the young heir to some fortune, raised by his grandmother. I'm thinking that his parents were killed off by the shapeshifters.
Anyways, the kid is mute. He expresses himself through paintings (he's really good). He draws one picture a lot: himself, drowning in a lake, from different viewpoints, sometimes with another 'him' watching.

I'm not sure if my "major twist" at the end would be too much of a surprise: he's a shapeshifter. The real kid died as a little boy. The shapeshifter was his friend and watched him drown (fall through the ice or something). The grandmother was concerned and made it turn into a copy of the kid and take up the lineage so she could gain one thing or another.

So is it too obvious? Please tell me what you all think! Thanks for reading this! (I know it's kind of long but I hope it was worth it.)

I don't think it's obvious. I actually like the idea.

But Sir Icehawk above me is right; it really does depend on how you write the build-up to the reveal.
Just really make it as subtle as possible so your readers might not think of it, or give false clues to trick your readers.

I like the twist, but why put it at the end?
Because the character dealing with the truth is much, much more interesting.
You should read Marathon Man by William Goldman. 3nodding It has an amazing plot twist that's really well done.