I have only 4 chickens, 2 that are kept in a dog carrier until they are large enough to join my two 11 week olds. In the dog carrier I have pine shavings that I clean 2xs a day. Now that they are 6 weeks old, during the day they are kept outside on the grass in a 3' x 4' covered dog pen. I hose down their dropping 2xs a day.
My two 11 week olds are kept in a covered dog kennel with commercial grade #30 sand floor. I use a screen poop scooper 2xs a day. They are the cleanest because the sand method is the best at keeping the chickens clean and living area order free.
The pine shavings is the hardest to keep clean and if they walk on their droppings, it sticks to their feet. Early on I learned to keep their feet clean as the pine shaving/pooh mix can wrap around the toe and when it dries can cut circulations. So daily, I rinsed their feet if needed and make sure their bedding is clean. I wear medical gloves--$9 for 200, and pick out the droppings. Then, once a week I discard the bedding and wash out the cage.
I keep their cages clean whether I am bed ridden, sick or not.
Disease comes from unsanitary conditions. This is not safe for the chickens, you or your son. You need to rethink your chickens bedding.
If your chickens live in clean conditions, I am sure that will eliminate them from being covered in pooh. I rinse my chickens feet outside with the garden hose and pat dry afterwards. Not sure about wetting down chickens to clean them...I will leave that to those who have more experience. But as far as keeping their area clean should eliminate future messes.
Also for space, each chicken needs amble room. In my case, I would not exceed 50 sq feet for 4-6 chickens. 4 is perfect as 6 would be cutting it close for 50 sq feet of space.