Something is amiss in his digestive area. I definitely would not wait for the appointment in three weeks; especially since he is up there in years.
First be sure it isn't a tooth infection. If you smell into his nostrils and you can't smell anything, it most likely is not an infected or abscessed tooth (even though he only has four there could still be problems).
Also, smell his breath. Infection OR a stomach sour smell will be noticeable if something is wrong. The severe sour smell was apparent in my horse with hind gut ulcers right before he colicked. Now that I know better, I am always smelling his breath.
That all being said, your horse can still have something very wrong somewhere in the digestive tract or vital organs (including spleen) and still not have bad breath. Which I why I don't think it prudent to wait three weeks for your normal appointment.
Is his stomach on the bloated side? How about his sheath - does it appear puffier than what is normal for him?
One of my horses had what I refer to as "cow bloat". He also had so much methane gas that, not only could he have blown up the barn, but I could hear him 1,200 feet away, on the ridge
The vet's best guess was an upset in his liver or spleen. In my case the horse was put on prescription herbs, which fixed him.
Since it turned out that three of my four horses ended up with some sort of digestive problems, I can only conclude the vit/min supplement they were eating had fermented. I keep it in the house and the expiry was 10/13 but nonetheless I noticed it had a strong alcohol smell to it.
Is there a chance your current bag of ADM got damp and fermented? None of us look at the expiry date on feed bags but we should. Maybe it's outdated, especially if you're only feeding one horse and 50 lbs takes a long time to use up?
What about hay? Is it on the musty smelling side? Maybe some clover in a few bales that normally isn't there?
Just anything out of the ordinary, even though you haven't changed anything --- makes ya crazy under those conditions
At any rate, the manure has an excessive smell because something's going on internally - I can't believe it's the weather. I might believe change in seasons because grasses do change with the seasons but, weather doesn't change the smell of manure