The reality star is already a "doting future stepfather" to Chyna's 3-year-old son King Cairo but he's "always wanted to be a dad," according to a close Kardashian source.
And there's a big reason as to why he's elated to become a first-time parent. "Rob was very close to his dad and lost him at a young age. He's never felt like he had a real father figure," the source added. "He's always wanted to give that to his own kids. He's really a loving guy and will give a lot to his children."
Kardashian, 29, popped the question in April – just 10 weeks after the couple took his relationship with Chyna went public.
But the happy sentiments are not felt by all the KarJenners, who have mixed feelings about the new addition. Though their reactions do not come as a surprise, given that their history with Chyna, 28, has been a rollercoaster ride.
"The family found out in the last couple weeks. Rob told everyone separately. They didn't have a big family meeting or anything like that," a separate source tells PEOPLE. "When Rob started dating [Chyna], it was like, 'Okay, this is a phase.' And when they were talking about the engagement, it's like, 'But it won't last.' But now that there's a baby coming, everyone in the family knows that this relationship has permanence. Even if it doesn't work out, they're going to be co-parents of this child forever."
While Chyna's ex Tyga, who is King Cairo's father, has congratulated her on the baby news, no one in Kardashian's family has extended the same blessings to the mother-to-be, unless you count Kim Kardashian West and Kylie Jenner liking Chyna's baby announcement on Instagram.
"Kris isn't thrilled," the insider reveals. Another source echoes similar comments regarding the momager, telling PEOPLE, "When Rob and Chyna started up, it was really shocking that it got so serious so fast. [Kris] wasn't thrilled but of course she loves Rob and just wants him to be happy so she made an effort to get used to the idea. Kris is used to being in control, and right now, she's not, at least when it comes to her son."
Neither Khloé, Kourtney nor Kendall Jenner have publicly congratulated their brother, but a Kardashian source explains to PEOPLE the family's animosity towards Chyna will become less icy.
"No matter what the family feels about Blac, it's not about her anymore; it's about a new baby who will be their flesh and blood, and I think you'll see the entire family come together because this is happy news," the close family friend reveals.
"Kim is probably the one who is least shocked or upset about this. She's telling people that she's excited to be an aunt again," the source continues. "She has the attitude that this is family, you don't always have to agree with family, but you have to be there for them and be loyal to them. That's something that's reinforced by Kanye, who didn't grow up with siblings. He always encourages Kim to recognize the gift that she has in a large family."
So who other than Kim will be the most likely to warm up to Chyna first?
"[Kim and Kanye] believe that [Kris] is going to come around, because there is no way that Kris is going to freeze out a grandchild," the source explains.
"She adores all her grandchildren; she lives and breathes them. And this baby will be just as much her grandchild as North or Saint. And that baby will be surrounded by a loving family. Is anyone in the family a huge fan of Blac Chyna? No, not really. But they will love that baby so much. All of them will come around."