Try using Oxyclean in the wash, and wash and dry it two or three times.
Not all down is washable. I assume yours is?
Once I got a featherbed from Company Store. I had already ordered down comfortors and down pillows from them, which are really nice. They aren’t the cheapest down products, but shopped the winter clearance sales (they have a summer sale too). Its been well worth it because I have had them for years and washed them regularly and they are still in excellent condition.
A feather bed is like a huge fat comfortor that you put on top of your mattress and you sleep on. So now my bed was the ultimate of luxery - like sleeping in a big puffy cloud with the down comfortor and pillows, too. I loved it. But, there was this smell.
I thought perhaps because it was new. So I washed it, and dried it. I had to take it to the laundrymat to do that, because it was huge. It still smelled. I took it outside in the sunshine for two hot summer breezy summer days, bringing it in at night. It still smelled.
So I called Company Store, and told the woman there about my many attempts to rid it of its pervasive smell, and she said that feather products do have a certain odor. (Its like when you handwash a wool sweater. It has a certain distinct natural smell). She said if I could not live with that particular natural odor they would be glad to take it back. And they did, even though by the time I called them it was several months after the purchase. I think they even paid shipping.
So, I wonder if your down comfortor is part feather? If you can feel the stiff part of the feathers when you scrunch it in your hand (whereas down feels all fluff), then it has feathers with the down. Because if your son hasn’t had a problem with stinky bedding before, then maybe it is the down (/feather?) comfortor.