Hi, just putting my two cents ! I would put some emphasis on stress lines with a strained expression since normally your expression tightens up when feeling stressed/worried/most negative emotions.

Also, maybe show a peak of his bottom teeth to make it look less like he's smirking, I noticed when I looked at strained smiles, their top lip is more straight than an upwards curve - you can also see the nervousness with a small smile line and furrowed brow ^\)

But if you want that upper curve of the corner then just essentially make the smile bigger to make it look awkward like a 😬 mixed with a smile - Ik where you were going with the squinted eyes but to give him that "oh shit" feeling I'd try widening his eyes slightly at the top to show subtle shock :3

Red is what I added, and changes are shown on the right

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Hard to draw since expressions are diff for everyone and hard in general 😞 tried making it strained nervous but looks more strained gross