Rhys also reveals that he's been speaking to son Sam in only Welsh

When it comes to their new addition Sam, Keri Russell is as cool as a cucumber – but dad Matthew Rhys? Not so much.

The actor opened up about being a first-time parent during an appearance on Live! with Kelly on Monday, revealing that he’s not nearly as seasoned as his partner, who has two older children from a previous relationship (son River, 9, and daughter Willa, 5).

“I’m the one going, ‘What about…’ and she goes, ‘Shut up it’s fine,” Rhys, 42, said.

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Sam was born last May – the first child for Rhys and Russell, 40, who costar on FX’s The Americans. PEOPLE first confirmed the pair’s romance in April 2014.

“There was a moment recently when – I said, ‘He has a very heavy cold and we’ve got to take him to the doctor,'” explained Rhys. “She’s like, ‘He has a cold, I’m gonna get my hair blown out.’ I go, ‘I’m gonna take him to the doctor.’ She went, ‘You take him to the doctor and when the doctor tells you he has a light cold, text me.’ ”

LIVE with Kelly
Credit: David M. Russell/Disney/ABC Home Entertainment and TV Distribution

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As predicted, Rhys said, 9-month-old Sam had only a light cold.

Rhys said that Russell’s kids are “brilliant” siblings, adding, “they’ve claimed him as their own.”

Meanwhile, the Wales native is focused on his son picking up his native tongue, revealing: “I’m speaking [to him] purely in Welsh.”

Russell confirmed this during her own appearance Thursday. “We have no translator! They’re speaking in their own language.”

Joking, Rhys added, “His replies, you would think he was speaking in Welsh as well. Quite a guttural language!”