Maybe there was a formula change or something wrong with it that he can detect but you cannot. Try a different feed.

Also, 5 or 6lbs is not nearly enough complete feed for a horse who is not able to digest hay at all. Complete feed is intentionally lower calorie to try to get more fiber in. You'll have to calculate the amount of calories per pound of feed and feed him enough to get ALL of his calories. It's usually around 15k calories per day needed. Most horses can only handle 5lbs of feed per meal so you might have to split it up into several meals. Sometimes adding an alfalfa pellet to the mash will make it more palatable.

This is a doable situation if he lives at home with you or you live 10 minutes from the barn. There is no shame in letting him go if you cannot manage this. It is HARD to maintain a horse who cannot eat hay, his body is already starting to let go. I would do it before winter unless he makes a serious turnaround.