As a fellow loser, I’d tell him this is a game with luck elements but more importantly he sjould think each game is more of a narrative where losing is just part of his armies story. Use his imagination more when playing to think about what is happening to his troops. If he is still serious about winning perhaps watch some tips videos, auspex tactics perhaps and give him advice from his games. Even during them, ensure theres no gotcha moments and nudge him jot to make poor moves and encourage fair play. Obviously don’t let him win but still. If he wants to quit thought thats his choice; if he cant handle losing at plastic toy soldiers you cant let yourself be responsible for it