Mats and a small bed ie 6ft x 6ft or smaller. Only works if he will only poo and wee on the bit of bedding. My old mare was horrendous box walker but was fussy where she pooped and peed luckily.
Looks positively clean compared to the mucky horse at my yard. But photographs often don't show how bad it really is. I use wood pellets but I find that even these don't help much with a really mucky wet and messy horse. He uses four whole sacks a week and his bed still looks minging.
One of mine is like this on sparse bedding. Have you tried really deep straw? Mine go in knee deep loose straw and are much cleaner, poo falls through and stays in neat piles waiting for me to pick out.
You should try having my gelding. His bedding is completely taken out every day because he's simply too horrible to deep bed. He soaks the stable in urine, poos a ridiculous amount, stomps around kicking his mess everywhere, and is a genius at finding ways to detach his water bucket from the wall and throw it on his bed for good measure. He also has to have absolutely all of his brushes and rugs removed from the stable or they either end up thrown across the yard or trampled into his bed. I've tried a small bed and it ends up utterly wrecked by morning, really deep straw, and everything in between. I daren't even attempt shavings because he'd go through them so quickly that he'd bankrupt me within a week. No useless advice there, admittedly, but at least you know I can sympathise!