Crib wedge

Arroya
Hey ladies ! 
My LO has been diagnosed with acid reflex :( doctor recommended to elevate his head with a wedged pillow but my LO slides off & down & by the time he wakes he is horizontal instead of vertical. Have any of you used one & how can I keep him laying the right way I'm nervous he is going to hurt himself or get trapped in his blanket because he is always swaddled.