Hi Corrine.

Been through times like these.  You can do only what you can do. If he's not having it, let him and you rest and try again tomorrow.

Go make a cup of tea or a glass of wine and take a mini vacation.

FACT: Iron overload has been identified as a significant factor in IR horses that are not receiving a mineral-balanced diet. (Nielsen, BD, Vick, MM, Dennis, PM, 2012. A potential link between insulin resistance and iron overload disorder in browsing rhinoceroses investigated through the use of an equine model. J Zoo Wildl Med. 2012 Sep;43(3 Suppl):S61-5.)
See  E. M. Kellon, VMD, Iron Overload and Insulin Resistance, 2013 NO Laminitis! Proceedings, Equine Cushing's and Insulin Resistance Group Inc.