I don't have experience with interface builder. But I've done C++ based desktop app with QT Widgets which has a similar concept and implemented my UI using code and I also implemented some websites. I completely disagree with Basecamp. When you design you are basically using a divergent thinking process: you are discovering, thinking and creating many ideas/ solutions for problems on the fly. This process is already hard when you are not worried about code (it consumes willpower and memory from your brain). Good design is about "removing stuff" and I change things several times. I don't design only the "skin" but the flow of steps to complete actions and I think that code is too slow to create that many iterations and ideas. Sometimes I after I place 3 elements in my canvas, I find a way to kill them with one or even the entire page, but if that was handed to the dev that would make him/her frustrated because they just got that built. I'm also a believer that tools puts constraints in your process when you code you are worried about making it work so you might avoid naturally certain solutions (because it would cost too much time), who needs another bootstrap implementation? But what if that new idea or interaction model would make a huge difference for your app? Think about the Tinder Swipe. When I do the design I forget about code and I focus on the best solution for the user, which could be even beyond material or iOS design guidelines. If you check Fantasy's work they have better interaction models for some apps than current trends and those guidelines I just mentioned. https://dribbble.com/shots/3030037-Seat-flow-for-Tinder-Travel-concept-by-fantasy
Now, I do think that is important for the designer to implement and learn front-end programming to do better design. Going back to the Tinder example, the guy who created that (Jonathan Badeen) has a background in both front-end dev and design. I think that code really helps you design better (if you are interested).
Now, potential solutions to your problems:
1-To create an incentive for designers to code you need to find a specific type of designer. Some designers are artistically inclined, in game design, they are called artists. These folks might better take other paths to improve such as doing marketing design, illustration, branding, or moving into animation, motion graphics and videos. Another type of designer is more logical oriented like me, I'm interested in the big picture of the system, and how the logic of the front-end works in relationship with the user, what kind of widgets are better for certain problems but still fascinated about the visual aspect. This is the type that would be interested or someone who is already in front-end to willing to learn design. I think that this is more about personality than anything, just like you won't convince a developer to do repetitive accounting work even if you pay him more.
2-if you believe that your guy is more designer than an artist, he needs time to focus on the visual aspect and not do the two at the same time. If your process is too rushed it will be too much stress to switch in between the two disciplines. I might be wrong but I think that the process of programming occurs on another part of the brain, so when I code I need to focus just on that and I don't want to open photoshop, and the reverse also occurs. So allow your designer to have time to finish his screens first.
3-Pay him better. If you want a designer to perform other duties such as managing people or coding, don't expect him to do developer's job but making half of the salary.
4- Stop using photoshop. I've used photoshop for 15 years and so far it's my favorite app of all time, but it's not the current best tool for UI work. It was developed to be a photo editing tool but it became too bloated. If you are MacBased use Sketch if you are Windows based use Adobe XD. I recently switched to Adobe XD and my workflow is 10x faster. It's vector based, it removes the need to use layers, it's easier and faster to use and way faster to export, not mentioning that you can connect the screens and make clickable prototypes. Adobe XD can export in different PPI's and it has also a developer's hand-off add-on: https://www.youtube.com/watch?v=Jdp8A4Vmt4g