Please read about magnesium deficiency in horses. But you will have to put forth the effort to make sure he gets the supplement twice a day.
Also going to talk to the barn owner and see if she would be able to get him from the pasture some how twice a day to do it, even if I gotta pay extra. I really can't make it out more than a couple times a week. My son is only 8 months and I can't take him with me during the day, no ones home with us till later at night. Also with a pretty fixed income with me not back at work I can't be driving that much there everyday. My horses will both be back home within a month or two though so Ill be able to feed them and give them whatever they need and whenever they need it.