They might be tough guys, but they're also softies.
"Rosie and Jason are so fond of Sly and he's like their baby guru," a source told The Mirror.
"The chat always seems to be [about] parenthood," added the source, whenever the "Rambo" and "Rocky" star visits Jason and Rosie's Beverly Hills home.
"It's not a sight you expect — two action heroes talking about [diaper] duty over a beer in the back [yard]."
Yet that's what happens. "[Sly] has said 1,000 times he'd love to babysit their child," added the source. "They'd take him up on it too. He's one of the few people they'd feel comfortable leaving their baby with."
Jason is so impressed with Sylvester's parenting advice that if he "has a snag with raising his baby," said the source, "he'll probably call Sly before he Googles it."
Sylvester has five kids: sons Sage, who died of heart disease at age 36 in 2012, and Seargeoh, who has autism, with first wife Sasha Czack; and daughters Sophia, Sistine, and Scarlet, who all served as Miss Golden Globe 2017, with third wife Jennifer Flavin.
"He's warned Jason how hard it is to be a dad to girls," said the source. "So it's not only advice on newborns but teens too."
Rosie, 30, confirmed her pregnancy on Feb. 9 with an Instagram post of herself in a bikini, her growing bump on display, with the caption, "Very happy to share that Jason and I are expecting!! Lots of love Rosie x."