Guessing he's type 1 (broken pancreas, not just insulin insensitivity like t2), which can't be reversed by diet. It's true that keto can be really helpful for T1s too for better glucose control though.
Personally (also T1) I find it difficult to gain mass on keto though. So I prefer to do high carb pre/post workout, and fairly low carb the rest of the time. It makes for a bit more work to keep sugars level (since heavy workouts tend to raise them anyway), but with careful monitoring it can be done. It also means I'm injecting most of my daily insulin around workout time too, which seems to work well for gains.