Feed him raw.
For some strange reason, Duke was really hyper the first part of this week. He was bouncing all over the place, waking me up in the mornings, throwing tennis balls in my lap, etc. This was pretty normal behavior for him when we first got him, but definitely uncharacteristic of him in recent years.
For the past few days, I've been giving him rice because he wasn't having solid bowel movements. I just quit giving him rice, so he's back to an all raw diet, and he instantly calmed down. I guess he's just really sensitive to sugars/carbs. I had noticed the pups calmed down a lot when I switched them to raw, but I had attributed that to age. Coral had calmed down a lot around that age so I assumed Duke was doing the same. Guess I was wrong.