Hi,
Well couple of bits of info to check first to try and see if it can be narrowed down a bit.
When you say red how red exactly? Mild blush pink or blood red or somewhere inbetween?
When did he last shed? What are the temps and humidity and how are those measured and the cage setup? - this way we can try and see if anything stands out.
Did you change anything lately? How fast is it spreading and can you get any clear pictures of it?
Hopefully that should help folks narrow it down - but it might also be worth seeing when you could get a vet appointment if it is something your sure is serious.
Any other questions/ideas guys?
dr del