The cause of Arthur's lack of weight gain - he is dropping a lot of feed again

For people who don't know us, Arthur came to us in terrible condition, he has gained weight but doesn't seem to be putting on any more. He's stuck.

Arthur escaped his paddock and I fed him separately from the mares and was shocked to see how much feed he is dropping. More than he was dropping when he came to me (I think - not always objective in these matters). He did drop significantly less feed when the dentist came, but it's deteriorated (which I find strange to happen so quickly).
I'm doing some experiments with feed types and wet and dry feed to see if it makes a difference and also to see if adding chaff makes a difference.
I'll also get a second dentist out for a second opinion. The first dentist didn't think he could do much to help and that the reason for his condition was his teeth. It looks like he is right. Hoping another dentist may have some more ideas.

Edit, I'm not thinking, after I fed him this the bowl ended up on the side of the embankment. I will forgive myself this time, as I've been ill and I think it's affecting my capacity to think!

#RescueHorse