"Don't be sad for Jimmy. Feel sorry for the doctor and the nurses who have to keep him from getting up and going to work," says Gerald Rafshoon, who has been in touch with the 90-year-old Carter since the former president's stunning announcement on Wednesday that doctors found cancer in his liver that has spread to other parts of his body.
"He's in good spirits – feisty, even," says Rafshoon, who served as Carter's White House communications director but has been close with the Georgia Democrat since his first (and failed) run for governor in 1966.
"Nothing about Jimmy has changed with this diagnosis and I cannot believe that Jimmy Carter is going down for the count anytime soon. Now he just knows what this next year's book is going to be about."
Carter, a prolific author, painter and global activist in his post-presidency, was out with his latest bestseller – his 29th book – just last month. And while this one is titled A Full Life, Carter still has much to pack in, says his cousin Betty Pope, a close friend of the former president and his wife, Rosalynn.
"They'll get through this," Pope tells PEOPLE. "And I'm sure he will go as long as he can and continue to do what he's been doing. They're already planning a big family Christmas with the kids and grandkids – 27 of them in Plains all week."
Carter, who, with Rosalynn, splits their time between their modest home in the tiny, rural town of Plains, Georgia, and an apartment in the offices of The Carter Center in Atlanta, heads back to Emory Healthcare hospital in Atlanta next week to start treatment and get a clearer picture of his prognosis. He has a devastating family history of pancreatic cancer, which killed his father, brother and two sisters. Breast cancer claimed his mother.
A statement from The Carter Center on Wednesday promised more details "when facts are known."
Meantime, Carter has enjoyed hearing from President Obama, Vice President Joe Biden and other well-wishers.
Spoke with President Jimmy Carter tonight. A great man, always upbeat and optimistic. We're praying for him.— Vice President Biden (@VP) August 13, 2015
Carter and Biden spoke for about 30 minutes and a source tells PEOPLE Carter "really loved" the conversation.
And those who know him best wouldn't be surprised if he was bending Obama's and Biden's ear about everything but his health.
"He didn't feel sorry for himself when he lost reelection to the White House. He's not going to feel sorry for himself now," says Rafshoon. "He'll find things to do, whether it's his next book or a painting. And he'll take whatever treatments come his way. He's upbeat and taking this all with humor and impatience."
Adds Pope, "He told me many years ago that he wanted to die with his boots on, that he did not want to sit in a rocking chair and wait to die of pancreatic cancer like the rest of his family."
"You'll see him out and about I'm sure."