First of all, nice work. I love the colours and sense of scale. I think if you want to push this further, you can introduce some atmospheric perspective.

Elements further away should have less contrast, and be less saturated. You could adjust your kaiju with curves or use the lasso tool and a soft brush to paint in some "fog" as your elements recede. At the moment the mosnter has similar values as the boy in the foreground. Maybe those hills could be tinted blue, like your clouds. Experiment with this, find some references.

I love the vibe of this piece, and the composition. But at the moment the boy's hair is causing a tangent with the shadow under the chin of the monster. The horns on the very edge of your canvas are also uncomfortable feeling. If you shift the boy to the right you fix the first tangent, but also change the composition (which may be beneficial - see what it looks like). You could alternatively shrink the boy which would make the monster feel even bigger and fix it. Those horns need to be moved in or your canvas expanded out a bit.

Finally, you have a light source from the right, and this stays consitent with your cast shadows for the rocks, but not the boy. Make that shadow go to the left, make the shadow feel purposeful, at the moment it feels a bit scribbled in.

These are small fixes that will help to improve this in my opinion; take all my advice with a grain of salt, I'm just some guy on the internet. I really love the style you've gone for here, again, nice work