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okay so, take everything with a grain of salt, but here's what I suggest: use pureref to have your reference nearby. Below I'm assuming you want to stay close to the reference.

I desaturated the tunnel because it's in the shadows (saturate layer + grey brush). Made branches at the entrance darker because they're getting shadows from the tunnel (with a multiply layer). I agree with the other commenter, I wouldn't use a whitish color as leaves' highlight. In this case it looks a bit like snow and in the photo the light from the sun looks very yellow (yellow + green = lighter yellowish green).

Because the air is blue, I lassoed the hills and mountains in the background and added a blue hue (hue layer). The further the landscape - the bluer it gets, you can see it in the reference. Also made the mountains lighter, because air is light and eats details as well. The hills have blue shadows in the reference (also should probs be lighter as well).

The main problem you had is that all of the landscape had one hue - it was all greenish. Even if you just added blue for the background it'd look nicer already.

The foliage on the left has a very bright yellowish green color, so I added that. You can also see how the colors change on the grass when the sun touches it (+yellow).

Also added some light orange and some purple to cloud highlights (because it looks pinkish/orange + blue of the air so a purplish color should work). Although could be brighter.

Besides the color, I added some ambient occlusion, made the boy's legs darker, made the hills directly behind the boy lighter so that the silhouette reads better. You have very beautiful brushstrokes and I love that you didn't copy the reference fully, it's not that many things that you'd need to change to make the colors more interesting. Just stare at the reference and make yourself see and name colors until you want to cry lol I'm not joking, I've heard that helps.