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"Koa has some big news to share with you all!" Kian captioned the clip, which he posted on Facebook. "Listen carefully now!"
"Mummy's got a baby in her belly," says Koa in the video. "I'm going to be a big brother!"
The couple, who married in 2009 and welcomed Koa in 2011, previously spoke exclusively to HELLO! Magazine about their plans to have another child.
While Kian was taking part in I'm A Celebrity... Get Me Out Of Here in 2013, he realised the extent to which he missed his wife and son and how much he wanted to expand their family.
Kian and Jodi spoke to HELLO! about their baby plans last year
"We were in the canteen just after he'd emerged from the jungle," said Jodi, "when he looked at me and said, 'We've got to have another baby!' I was stunned – but I'm all for it. I'd like a little girl next."
"Being away from my family made me look at our lives," added Kian. "I was on top of the world and the only other time I'd felt like this was when we had Koa. I'd love to have a mini Jodi. I'm so lucky to have her and Koa and if we can give him a brother or sister, why wait?"