:iagree: Selling this horse will subject him to the possibility of a miserable end. Often people don't want to make the decision to have them PTS, and especially because its not free.You're in a boarding situation and not able to afford two horses. The vet bills and meds on this horse will be costly.
The horse has no future once he leaves your care. My grandad had a pony with heaves so I know how it can progress no matter what is done for the horse.
If you know for fact the horse has COPD and is in an advance stage, do the right thing and have him euthanized.
It isn't easy. Nobody wants to do it but it IS the humane and ethical thing to do.
I have had to lay five horses to rest in my lifetime - I was in a boarding situation when I had the first two PTS'd from cancer and ringbone at ages 29 & 27. It isn't like I don't know what I am talking about.
Your choices are to throw money at him to make it comfortable for him until it is cruel to keep him alive, or to have him PTS in which case you know he has been cared for till the last, and gone peacefully.
Have the vet out, and get advice as to his long-term prognosis/care and when vet thinks is a point of no-return.
It is a hard decision, but only you can make it.