This past weekend a male friend of mine said that he had oral sex and now his scrotum is itching. what could this be?

No way to know.: Without direct visual examination, along with a microscopic scraping and perhaps a culture, there is no way to tell what is going on. Your friend will need to see his doctor or a dermatologist.

Pretty much...: ...Anything, from crab lice, to hypersensitivity to saliva, to herpes, to underwear being too tight. Might be worth checking with a doctor if it's severe.