Although epsom salts are a magnesium compound the magnesium isn't in the right form to act as a calmer. They're far more likely to irritate his stomach & give him diahorrea! I know they were an old remedy to calm an excitable horse but I think they worked mainly by making it feel slightly ill.
If you don't want tobuy a "proper" calmer try CalMag or Mag Oxide.
I tried Magic & it didn't work very well on my horse either. I find Feel Good 30 Science Formula Calmer works well for mine & most of the time I don't even need the full dose. I only tried Magic because it was easier to get hold off but quite quickly decided it was safer to order FG30 on the internet!
I do give all of mine about a teaspoon a day in their food, and double that to the really lively one. It does seem to work even if it is the 'wrong' kind of magnesium and doesn't make them squitty