Looks like you probably didn't do too badly, but this is a job for someone with lots of experience in rehab, not just a 'backyard trimmer'. So saying, I realise that in many parts, good trimmers/farriers are impossible to find. If you're in that boat, I'd keep looking for one tho, as well as studying all you can! E-hoofcare.com is one place that will help you help him. As NM said, we need more/better pix & info to give much specifics.
The big cracks were probably due to long term neglect & were really bad at one stage, and haven't(possibly can't) grown out due to continued inexpert, imbalanced trimming. His feet are flared all round, especially the quarters, and need some serious relief with strong bevelling. If there is any chance of any infection in those cracks, it would also be prudent to open them up, although resecting is another thing that is something for an experienced rehab trimmer. As the cracks appear full wall length, if they need to be opened up a substantial length, the hooves may need to be braced across the face of the cracks.
The constant rings on his feet, chronicness(is that a word?) of the flares and that you say he has flat soles means that chronic laminitis is very likely a big part of the problem too.