The source adds to OK! that 13 years after their split, Brad and Jen discovered the "passion is still there" during a covert reunion a few months ago.
"They've been seeing each other ever since," says the source.
"Brad has stayed at Jen's house in Bel Air - he comes and goes through a secret entrance on her property - and even when he's on set, he calls constantly, sometimes just to say he loves her."
"The spark is alive and well," the source adds.
OK! reports Jen even made an undercover visit to Brad's Los Feliz mansion and "played video games and watched movies" with Brad's six kids.
The source adds that while 12-year-old Shiloh "told Brad she really likes Jen," Angelina was much less enthused.
"She threw a fit!" a friend close to Pitt said.
Once Brad and Angelina Jolie settle their bitter custody battle, the plan is to "fully move on, and to let the world know," OK! reports.
Brad Pitt is yet to respond to OK!'s report.