I know this may sound really odd and gross but [medical] maggots work the best... the eat the dead flesh and really clean up the wound...within days you'll see a huge improvement. I would definitely look into this as disgusting as that sounds.
Other than that suggestion... soaking his paw in epsom salt and hot water should help. I would try to stay away from peroxide since it burns and also damages some of the good flesh. Perhaps try betadine/iodine instead...