The American Idol judge confirmed his engagement to make-up artist Mezhgan Hussainy earlier this week.
The show's stylist Miles Siggins, who is reportedly also a good friend of Hussainy, has now told Fancast that he has heard the ceremony will be held this autumn.
"He seems really happy with her," Siggins added. "I have to be honest, though. I was kind of surprised. I knew they were dating, but I was surprised that they got engaged [so quickly]. But she's great. I guess he just made his mind up. And they are really happy together."
Siggins also claimed that Hussainy's engagement ring cost $400,000 (£260,000) and suggested that the show's host Ryan Seacrest will be involved in the ceremony.
Cowell's spokesperson refused to comment, saying: "Simon's private life is his private life."
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