He looks sodden. Do they not have shelter or did he just not go inside? What happened to the feathers on his back? It looks like you have feather picking going on, which is often associated with a lack of space .... what starts out as loads of room when they are chicks suddenly is no where near enough when they hit adolescence, especially if you have a cockerel or worse still more than one, as well as pullets.
Wet conditions, particularly warm wet conditions, are a breeding ground for coccidia and bacteria.
Can you post a photo of your whole coop set up so that we might suggest things to improve it.... perhaps a tarpaulin to shield the worst of the rain would be a cheap solution. I know prolonged wet weather can be particularly taxing when you have animals.... I live in the UK so I know about rain and mud although we have been incredibly lucky the past year or so in that respect! I rake up dead leaves from the roadside and lawns at this time of year and throw those into the run to help dry up the mud and prevent the build up of unhealthy organisms..... literally inches of leaves, coarse wood chippings or bark. Layer it up nice and deep. Think about it as creating a compost pile in your chicken run. It is called the deep litter method. Composting involves green and brown materials.... green provide nitrogen and brown provide carbon. Chicken poop is considered green because it is high in nitrogen and you need lots of brown carbon rich litter to soak it up. Dead leaves, dried out grass cuttings, straw, wood shavings, cardboard, shredded paper etc. In wet climates the larger aggregate wood chips and bark help with drainage. With deep litter you only muck out once or twice a year and the compost you get out can go straight on the garden.
