Rob Kardashian: Shirtless Cookout with Blac Chyna

Rob Kardashian's confidence is back ... and we know this because his shirt was gone during a backyard hang with Blac Chyna.

Rob stepped outside half naked ... giving us a first glimpse at his bod since he started his diet and fitness regimen. Chyna shot video of him Wednesday afternoon as he peered down at her grilling a ton of grub.

As we first reported ... Rob's already down at least 40 pounds, but it's been hard to notice, since he's still sporting a lot of baggy clothing.