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 MagRestoreŠ, 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.