Don't be too alarmed. It could be any number of things. I would think that if a person had cancer or some other serious illness, there would be more symptoms. I would think the cancer would have to be very bad for you to smell it, and yet, I have never noticed a smell from cancer patients until they started chemotherapy.

There are so many things that could cause a change in smell. Change of diet, environment, activity, etc. Those changes in smell are much more predominant and easier to notice.

The sour milk smell sounds a little like he has been perspirating and hasn't showered since then. Maybe he has picked up exercising or his job has caused him to be more physical.

Whatever it turns out to be, I hope it works out.