of course! From what I gather, mg deficiency is pretty common in the soils, so supplementing it is a good idea, maybe going by the tell tale signs, full dose, once he shows loose stools, his storage is full. I'd get MgOx or MagRestore©, to be sure it's straight mg. To be absolutely sure, you'd have to have a hay analysis and supplement according to it.
You could also try CoQ10, 300-600mg for two weeks, then 100mg daily, it's an anti oxidant, will take care of any inflammation. I use the people stuff, just open the capsules and mix the" creamy" stuff in the feed.