"His action caused him to be alone despite being in a throng of people."

The common expression is "alone in a crowd". I'm not sure how he could own the crowd (throng)? And a throng is naturally large and forceful, so no need for "big". "Alone" and "throng" also balance each other, so no need for "despite". "His action made him alone, within the throng."

"His actions made him alone in spite of his big throng." i believe you are trying to say, all you did wrong was miss an "S" after action and added an "E" on him. i don't know if "throng" is what you want to use, maybe you want to say thrown?

Not exactly. His action caused him to be alone, despite his big throng. ...I'm not even sure what this sentence is intended to mean.