Hoof Problem

Okay. I'm very mad. I have told my trainer several times not to put y horse out in the mud/rain/ect., because he has weak hoofs right now(we're working on making them stronger). I came to the barn today, and he was inside. It has been raining for the past 3 days, and so the pastures are a mess (we don't have much grass in our pastures anyways, so they're mostly dirt, and get REALLY muddy when it rains). Anyways, I looked at him, and it was obvious that he had been outside, cause he was covered in mud, so I took him out to observe the damage.

HIs back right hind hoof, which isn't shod, has a chunk (about 1 square inch) all bent up and cracked. I have been putting hoof stuff on his feet a couple times a week to make them stronger, but they still can't withstand the mud.

My question being. What should I do about it?? I didn't ride him, cause he seemed a little lame on that foot. Im pretty sure its bothering him. But, should I call the farrier? And how long will it take for the hoof to grow back? I'm thinking of putting him on a hoof supplement to help a little.

Thanks for your help.