That is not a lot of carbs. He is not going to get diabetes from eating oats, beans, and sweet potatoes. It’s too much fiber.That is a lot of carbs. Even if they're healthy, that's still a lot. Maybe eat more protein and fat for a few months and see if that makes a difference? Might want to try fasting too.
Could also be genetic. Either way i'd get a second opinion.
Carbs = diabetes is a gross misunderstanding of the pathology. Dude is 6’0 and 150 lbs. He needs to take the test fasted and should the results come back similar there is a genetic predisposition.