Earlier quoted context omitted.
Transliteration is almost always a mess, rarely yielding good results. If I were him I would just demand the authorities to spell my name as Simon. Or, perhaps, keep it as Semen and make fun of that myself. I could even sue someone who would ban me for this.
Have you tried demanding something from Russian authorities? They have a transliteration table and Семён is Semen, no changes allowed.
Proof: https://www.transliteratciia.ru/transliteraciya-imeni-i-fami...