Definitely high time your mums horse see's the vet I think OP.. Poor poor horse
He at least needs some pain killers of some description at the very minimum.
He's probably gone and coliced due to 1. The pain and stress of having such sore hooves, and 2. everything else that he's going through at the moment, including the weather.
How are you wrapping and padding his hooves? I would recommend incorporating nappies and duct tape into your hoof wraps if you haven't already.. Wrap as you normally would with your padding and vetwrap or whatever your using, then put a nappy over top (like you would put it on a child, do it up over his hoof) and duct tape over the nappy, believe me when I say that mix is brilliant, I use that method when dealing with abscesses in winter and it has always kept hooves dry for me thanks to the duct tape, plus the nappy provides a little more padding also.
Is it mud fever or thrush? I got confused in your post on that one
And is he on any hard feed at all? Just thinking at this stage he might benefit from a good quality mineral powder (preferably produced locally if possible, then it's made specifically for what your grounds are likely to be lacking).. And a good warm mash might just make him feel a little better while he's eating it.
Poor thing, I hope you get this sorted soon