This is related to the recent QOTM about the Rein Back. Sorry to say, we’re in classic Vending Machine territory again! Horses that “guess” just aren’t on the aids enough. Easy to diagnose, hard to fix, but you just have to be quicker and more innovative than he is.
For starters your instructor can help you by either giving commands well before you have to execute them, by substituting non “cueing” phrases, or as a last resort, by spelling them.
Beyond this, your horse must learn that he doesn’t know what’s coming next so he has to wait and find out what you want. Do the movements in unexpected places and for seemingly arbitrary lengths of time. For example, start a shoulder-in on the inside track when you pass V (rather than on the track at the corner letter). Then do it for five strides, gradually decrease the bending all the way to straight, make a half circle to the opposite quarter line and ride counter shoulder-in on that line during which you change the length of your horse’s stride. It means a lot of extra thought, but it will work.
If he’s okay at home but the issues recur in the show ring, you may have to blow a few rides at a schooling show and intentionally ride not to the mandated marks but to ones which require your horse’s full attention and cooperation. It’s kind of rude to say “You just have to be smarter than your horse.” They can get pretty darn creative in sussing you out if you give them even an inch of over-predictability!