Good idea with the pliers and tiles.
Chickens need about 2 grams of grit each week. It depends on how hard the food is and how hard the grit is so it could be quite a lot different than that much - either more or less.
They will not eat more than they need.
You can expect him to eat more at...
Can you get a sledge hammer? My sons worked at a rock testing lab one summer. They broke rocks with sledge hammers until all the material was fine enough to go through a fine sieve. You would need just a little and not that fine.
It would be much, much, much better than trying to work around...