On Wednesday, the reality star took to Snapchat to share videos of himself going hard on the treadmill during his second workout with his new trainer and fiancée.
"Day 2 with my new trainer Chy," he captioned a video of himself running.
He also shared a video of himself dripping with sweat and rehydrating with a cup of water after the workout.
"Rob, how was your workout today?" Chyna asks him in the video. "It was good?"
"Look at your shirt!" she says as he points to his drenched black workout tee.
Kardashian, 29, previously battled with extreme weight gain and depression, but since he took his relationship with Chyna, 27, public in January, he's turned over a new leaf, regularly working out and maintaining a healthy diet.
On Wednesday, the newly engaged couple announced on Snapchat that Chyna would be taking initiative to help her man continue to lose weight, training him for the next five weeks every day to whip him into shape for her May 11 birthday.
Meanwhile, Kardashian's family has remained mum on the engagement on social media and a source told PEOPLE Thursday that sister Khloé Kardashian feels "betrayed" and is "furious at the lack of respect." (Kardashian proposed to Chyna – who has a complicated history with the famous family – with a 7-carat diamond ring while the entire Kardashian-Jenner crew was vacationing in Vail, Colorado.)
"She can't believe Rob didn't tell the family in advance. She thinks Rob is causing drama on purpose and wants them [his family] to be arguing," said the insider, adding that Khloé, 31, plans on "ignoring the news" and won't be sending any well wishes to her brother.
"She is not happy about the engagement, and is not going to pretend that she is," said the source.