Is he just dropping it all over or slobbers too?
Some horses are messier eaters than others, but he's not old enough to be developing the old horse "spooge" that comes with loose lips, so at the risk of being re"dun"dant, might want a second vet take a peek in there.
Does he have large canines (behind the first set of teeth, in front of where the bit rests)? When the vet was out last week to do dental work, he filed down canines on one of my boys, and he's stopped dropping food since then, so I wonder if that could be it.
Not knocking your vet, but they are people, and I used to have a vet that was great at stitching wounds with comsetic brilliance, but I wouldn't let her touch my horses' teeth, but she was my go-to for other things. Perhaps you can find a second vet with a good rep for dental work? Some vets think you are supposed to file the teeth flat,but they need a bevel to grind properly, so your vet may have actually created a problem if he floated them wrong. Doesn't mean he's not a good vet,just that teeth aren't his strong point....
If your horse has always been messy, then that is possibly just his way,but I find unless there's something wrong or their old, they don't just drop food.