Very happy you've received some peace of mind. It's easy to panic when a vet gives you wrong information.
He will be just fine. I know others have said that but sometimes you need to hear it from a vet to rest your worries. That's natural. I do it all the time!
Everyone reaches a point where they need to learn something more than what they already knew. Don't beat yourself up over this. He will be alright and you learned a lot about horse feed and their needs. It's alright to question a vet and we love that you came to us for opinions and to seek out help.
I'm a bit surprised that the facility owner didnt step in when he/she saw the weight drop, and waited for you to say something before taking any kind of action... But I guess now you know that you have to pay close attention to how the facility owner runs things and ask questions or make requests if you horse isn't doing well, since they won't take action on their own.
And those who were seen dancing were thought to be insane by those who could not hear the music