Basically they need adequate protein, (likely low in just grass hay)the need low starch and sugar levels ,(usually best in most grass hay), balanced minerals such as magnesium, mangnese, and sulfer, and enough micro levels of certain metals particularly copper and zinc.
Too much calcium can affect the absorbtion of other important minerals so avoid straight alfalfa because that is very high in calcium .
So on a grass hay diet I would just add a good complete hoof supplement that contains the minerals and metals and methionene and biotin. And as long as he is not a insulin resistant horse, maybe add a LITTLE alfalfa to boost the protein. But not more than 25% or so of the total diet.