"One side of his face is paralyzed so he often has a lopsided expression."
"He has paralysis on one side of his face ~"
"One side of his face is paralyzed so he often has a lopsided expression."
"One side of his face" 「彼は顔の片側」
"is paralyzed" 「が麻痺している」・「麻痺があり」
"he often has a lopsided expression" 「よく歪んだ表情を見せた」
"often" 「よく」
"has a ~ expression" 「〜表情を見せた」"has a ~ facial expression" も言えます。
"lopsided" 「歪んだ」
"He has paralysis on one side of his face ~"
"he has paralysis ~" 「〜麻痺があり」
"on one side of his face" 「彼は顔の片側に」
[表情を見せた](https://eikaiwa.dmm.com/uknow/questions/93991/)についてもっと詳しく知りたい方はクリックしてください。
His face often looks lopsided because he suffers from partial paralysis on one side of his face.
この場合、次のように言うと良いでしょう。
ーHis face often looks lopsided because he suffers from partial paralysis on one side of his face.
「彼は顔の片側に局部麻痺があり、彼の顔はよく歪んで見える」
あまり失礼な言い方にならないよう、to look lopsided のように言うと良いでしょう。
partial paralysis で「局部麻痺」
on one side of his face で「彼の顔の片側に」
ご参考まで！
オーストラリア 
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