Your DN 6
Angry Birds Game is rated 3+ on play store
Manufacturers say in game adverts are suitable for 13 +
They way you described the advert, it sounded like a Freddie Kruger type deal. It's a clay animation, instead of banging on about complaining, you need to ensure the kid isn't on his own whilst gaming, and if possible buy a version of the game that can be played offline usually via in app purchase. You also need to talk to him and ensure he understands that it wasn't real, and is probably equal to, or like the cartoons he watches. Put some perspective on it.
I bet it wasn't even the clay animation that scared him, but the loud music. That may have been jarring because he wasn't expecting it.
I mean I get why you want to complain, because it says it's suitable for 3+ but the adverts are 13+ so either it should be a 13+ app, or have kid friendly adverts to make it 3+, but really it's a gentle YABU, because you or whoever was in charge of childcare should have ensured he wasn't alone, and should have read the small print. Or buy an in game, full version of the app.
And for the love of God, don't let him watch Watership Down!! 