40-year-old Robbie and Ayda, 35, are already parents to daughter Teddy Rose, who was born in 2012.
It has been reported that they have medical staff on hand as they wait for Ayda to welcome their baby boy.
A source told The Sun: "Ayda is due any day now. They are both excited to welcome a second baby into their family.
READ: ROBBIE WILLIAMS AND AYDA FIELD 'EXPECTING A BABY BOY'
"Robbie is especially excited because he is going to have a little boy. Ayda is terrified that they baby will come early like Teddy did.
"And her midwife has told her that second babies tend to come quite quickly. So they've got the gynaecologist on speed-dial."
But the couple aren't taking any chances and have prepared themselves for every possibility.
The source added: "They've got plans in place for a home birth in case the baby catches them out and a trip to the hospital isn't possible."
READ: ROBBIE WILLIAMS REVEALS HE WANTS A SON THIS TIME: "WHAT AM I GOING TO DO WITH A GIRL?"
Robbie has previously spoken of his desire for a son, saying: "Last time I said that as long as it was healthy, I didn't mind.
"Then we found out we were having a girl and it was a strange emotion. I was like, 'What am I going to do with a little girl."