Firstly Welcome to the forum and I'm very sorry to hear about your poor horse
The following is what we do with our rescue and it really works...
1. Keep the hay up to him 24/7, soaking it for half an hour before feeding is always a good thing. Absolutely NO grass at this stage.
2. Make sure he has a comfortable soft place to stand - somewhere with sand or packed hay so his feet are supported
3. Feed him SALT - plain sodium clhoride - around 2 tbsp a day
4. Magnesium is also essential so get a good mag supplement.
5. Do not give him any exercise UNTIL he is comfortable and then lead him out each day.
6. Feed PLAIN feeds such as copra and beet soaked together with added oaten chaff. Do not feed ANY equine 'fast food' as this is likely to be full of potassium which is a big part of the problem.
7. Get yourself a good barefoot trimmer to attend to his feet DO NOT be talked into putting shoes of ANY kind on him.
Do all this and his recovery should be quite short - he should be feeling a lot better in a couple of weeks. Zgood luck and let us know how you get on - photographs are always good as a record