Heaves , aka COPD is a ailment that makes breathing difficult for horses.
We all just came off a difficult summer of heat and humid, a time where many horses with heaves don't do well medication given or not...
Add in dust and allergens and many horses had some issues.
Sadly, my experience has been it never gets better and often you can not even maintain easily as this is a progressive ailment.
The horse in reality can breathe in but has great difficulty exhaling the air now stuck in the lungs.
Certain foods complicate and make worse the condition too.
But the horse losing weight in summer is not surprising to anyone who has watched the struggle...
The horse must be able to breathe to eat, and when it is so hard to breathe in/out they have no energy nor desire to eat...they just want to breathe, need to breathe to survive.
With cooler weather some of this may get better but this horse is better seen by a vet and vet guidance given on what to proceed with with food and approach so the animal can thrive not just survive.
Call the vet and ask your healthcare professional for assistance and guidance.