I am absolutely nowhere near as fast, but I also have leg problems that flare up even on 15 mpw.
First, is he seeing doctors and physical therapists? Are you sure it's just shin splints? You've ruled out things like stress fractures and compartment syndrome? What makes the pain better and what makes it worse?
I've been doing my research and trying to figure out how to best train around it. I think we can agree that two of the most important things runners need to develop are aerobic capacity/VO2 max and efficiency. Cross training can pretty much develop aerobic capacity just as well as running, and yet someone who runs 5 hours a week will be faster than someone who bikes 5 hours a week. This is because the more you run, the more efficient you become at running. Cycling and swimming don't transfer. Now, pool running might be different, I assume that's what you've been doing? Not swimming, right? I'm sure it's better than cycling but not perfect.
The good news is, you can improve running efficiency without running. Strength training, plyometrics, and sprinting are some ways to do that.
Here's a really great paper on cross training for runners and ways to improve running efficiency:
https://libres.uncg.edu/ir/asu/f/Honea,%20David_2012_Thesis.pdf
Don't underestimate efficiency. There's a study on how plyometric exercises affect distance runners, and the group doing plyometric exercises actually saw a small decrease in VO2, while the control group increased/didn't change. But the plyometric group ran significantly faster, because they had better efficiency.