The red butt and sneezing are unrelated.
Chicken skin that is bare and exposed to the elements turns bright red. It is not raw, just red. It is not an issue unless the other birds start molesting him due to the red skin being exposed. If he starts getting picked on by other birds and his feathers get plucked, then you can coat his butt in Blukote to stop it. Chickens are attracted to the color red and sometimes will pick at red butts. A quick shot of Blukote will disguise it, and they will lose interest. Reapply as needed. Blukote stains anything it comes in contact with, so he will be purple until he molts. That applies to skin and clothing as well so wear work clothes when using it. It can be removed from your skin with nail polish remover.