First, what are his living conditions? He needs as much turn out as possible on DRY ground, preferably hard ground.
Next, what is he eating? Diets high in sugar or starch can make chronic thrush worse. Consider a grain-free diet of hay, hay pellets, and vitamins instead of any horse feed or grains.
Adding Apple Cider Vinegar to his diet can help balance PH, making an less hospitable environment for thrush. Work your way up to 1/2 to 1 cup daily.
To treat thrush, I like to use a 25% bleach solution. It's cheap, easy, and works well for most types of thrush. Spray every other day until it's gone. If bleach doesn't work, then a mixture of sugar and iodine, made into a paste, works. Spread it on the bottom of the foot, boot, bag, or tape so it's on there for a good 30-60 minutes. Then let the horse just walk it off (don't rinse).