I can't help you with why the shoes are coming off but have you thought about barefoot? If there is no physical/medical/show reaons for him to have the shoes on he would probably benefit from some time with a good barefoot farrier.
I've had my trakehner barefoot for the last 9 months and his feet are absolutey flawless. My farrier explained when I got him that because he has really huge feet and he is huge, he is getting some stretching or something happening in his front feet that is making the feet a weird shape. Not noticeably weird but weird enough that it disrupted shoes. Since he has gone barefoot his general hoof condition is 100% aaaand since my farrier showed me how to keep their feet nice and gave me a file to do it with they have gotten even better. Barefoot to me, is the way to go. All my horses are barefoot. Other than my trak I have a tb and a 26 year old standie. My tb was typically flat footed and just had the worst possible feet. After 10 months barefoot she has 5mm of concavity to her feet now and the best thoroughbred feet I have ever seen and I've had a lot to do with tbs. My old man is well, old :) he has been barefoot for 9 years and he has the most solid feet I have ever seen on such an old horse. I put it all down to being barefoot
Just a thought