Backing in backwards helps. Do lots of work with other gates and smaller spaces. So he gets used to walking through narrow spaces. Do lots of groundwork and make his feet move before you try entering the stall. Make the stall seem like a safe haven. Lots of work outside the stall and no work inside the stall. Have him put a foot in the stall and then walk out, then 2 feet, then walk out and then 3 feet and then out and then 4 and then out. Make him move his feet until you feel he is annoyed and then put him all the way in the stall and stand their. Dont ask him to do anything, just stand. Itll let him realise it won't eat him
This is not meant as a joke: try sending him backwards into the stall, some horses don't feel confident to go into a stall at a new place for example. I work at a commercial stable and this has worked with a couple of horses.