actually, shell repair is probably not a good idea here.
if you tried to patch this section of shell you would likely end up getting the patch material on the snail's body, which could further injure it and could be fatal.
on the upside, he's doing great so far, obviously. the chalky section you're seeing between the intact shell and the mantle edge is calcification. he is laying calcium down directly on top of his mantle and organs since he can't grow shell fast enough to cover that area. it's a good sign of healing. watch that area and you should see it thickening over time.
in this situation, here is your best bet:
-be sure the water is pristine. don't change so much that you cause a new cycle, but keep an eye on the parameters and don't let the ammonia, nitrite, or nitrate get too high.
-add crushed coral substrate to the tank or put it in the filter, and get some Kent's Liquid Calcium to add to the water. it's important that the snail gets enough calcium to repair his shell. the more sources of calcium, the better. plaster-based vacation feeders help as well, as do "wonder shell" blocks. many pet stores carry these shell-shaped calcium blocks, and you can find them online as well.
-if possible, isolate him from other fish or snails. put him in a breeder net if necessary. not even shrimp should be with him, they can pick at his exposed mantle and lung.
-be sure you are feeding him high-calcium foods. collard greens are great, as well as fresh parsley, mustard greens, kale, and romaine lettuce. remember to blanch or microwave (for 10 secs) this food so he can eat it right away, as brigs eat only dead or decaying plant matter.
you're a great snailer to rescue such an injured guy. snails have an unbelievable ability to recover from injury and repair their shells. keep an eye on him, be sure he's eating well, and he should be fine. keep us updated, too!
