How do you normally worm him ?
If it's an oral wormer it won't be treating for external parasites and he may have gotten lice or some other nasties which would irritate the skin and cause him to rub/scratch excessively.
Are the patches diamond shaped ? If so, it could be erysipelas (sp?) in which case a course of anti-biotics will clear it up.
I'd get it cleaned and get some purple spray on it asap (I think it's proper name is genetian violet or something - it's used for horses, chickens etc) and if it's possible to cover it to keep the flies away that would be a good thing
If you can't cover it - dab tea tree oil round the sore bits (not on the broken skin or it'll hurt like hell) and the smell should help keep the flies off.
Hope he's soon on the mend - poor boy
Karen x