Dog howls every night

I got this dog about three months ago. He’s perfectly trained and a great dog, but the only issue is that he howls every night no matter what I do. I’ve tried taking him on walks before bed, giving him toys, bringing him into my room, and working on separation-anxiety training. He’s also fully awake when he howls, so he’s not dreaming. It’s just something he does. Does anyone have any advice on how to stop this?