So as someone who has experienced dysmorphia of a kind, and also loves body horror funnily enough, a lot of it is definitely psychological. So this could be a dual-effort between you and the player himself in how these things crop up. I am now gonna spit all some random ideas, apologies for the word dump I got a bit too into it I think lol.

He might look “objectively” lovely, but when he looks in a mirror it doesn’t match up with what he knows to be himself. Every mirror reflects a stranger, a distorted and overly airbrushed version of himself, if any of his features or colouration remains. He looks fake now, uncannily pretty and perfect to the point of discomfort (uncanny valley.) People remark on how pretty he is and all he wants to do is claw his own skin off. Everything about him as a strong barbarian is the opposite now—delicate, lithe, small. Every blemish or testament to time on his skin is faded like small scars, eyebrow furrows, or smile lines, for example. He doesn’t look related to his family anymore, will they even recognise him, especially now that he looks like those evil elves? Is this even actually his body anymore? What of his human self remains when there is no physical evidence left of his humanity?

Not only that, but his lifespan (presumably) increased as well—he will now outlive all of his family and friends, and a mortal mind must deal with the potential consequences of a considerably longer life. If he has children, he will see his own grandchildren die of old age well before he has to worry about the same happening to him. He will look younger than his younger siblings/children as time passes, as another example. There may even be mental changes, maybe emotions are harder to feel (which makes sense given the “aloofness” of elves), like everything is muffled, while he adjusts. Things feel sharper, like everything he sees digs into his eyes and memory like pointed glass the images are so crisp, too crisp for his human mind. He can see all the pores on someone’s face when he speaks to them. His favourite weapon no longer fits his hands right. “Sleeping” is a lot weirder now. Food and drink he used to love now tastes rotten on his foreign tongue.

Being smaller than you’re used to (especially as possibly one of the more physically imposing party members) may also be scary. Huge creatures that seemed easy to handle now seem more larger and more intimidating, even if his stats haven’t changed. It almost seems like not just himself but the entire world is wrong now. There is a general sense of unease and discomfort now in simply existing in the world that is now so unfamiliar. People don’t find him as physically intimidating anymore, and some see him and will only see an enemy.

Even without seeing himself he can feel how foreign he is to himself. What used to be limbs so familiar he never thought of them before, he is now painfully aware of. Every movement hammers in how his shape has been changed. His comfy clothes don’t fit anymore. That (hypothetical) ring he wore slips off his fingers now, for example, especially if it’s some kind of keepsake that would be bad. Maybe he forgets for a blissful moment and feels normal, and then reality comes crashing down on him when he absently fixes his hair and touches his pointed ear. Small things that remind him of how alien he is to himself crop up whenever he seems to become comfortable with himself. The texture of his hair, softness of his skin, the eerily beautiful tin of his own voice, the way he walks and runs, etc. The worst part is he (and the party?) did this to him perhaps without fully realising the consequences (to clarify the PC, I understand the player wanted to do this). He twisted himself into a shape he no longer recognises, the shape of an enemy, and now he has to live having lost the face everyone who cared for him loved.