Here are a few possibilities, focusing on sound similarity:
* Σον (Son): This is the simplest and closest phonetic approximation, using the closest Greek letters to produce a similar sound. It's pronounced "Son," with a short "o" sound.
* Σάον (Saon): This adds an "a" for a slightly fuller sound, closer to "Shawn" in length. It's pronounced "Sa-on".
* Σωάν (Soan): Using the letter omega (Ω) adds a different "o" sound, making it sound a bit more like "Shawn" but possibly less familiar to Greek speakers.
Ultimately, the best spelling depends on your priorities. None perfectly capture the original pronunciation, but these offer options with varying degrees of similarity. You could even use a transliteration and explain that it's a foreign name.