I am assuming sti stands for sexually transmitted infection. In the states they are just called sexually transmitted diseases. If the woman was infected with any std you could certainly get through oral sex. These diseases are transmitted through fluids and a woman's vagina gives off lots of fluids especially when it is being stimulated.
Get yourself checked out by a public health clinic or whatever. You have socialized medicine in the uk so it shouldn't cost you anything.
Ed c

Ummm, all sti's and std's are not transmitted through fluids. You can get some of them just from having contact with that area, ie warts, herpes, etc.

Hots'.
I didn't mean to imply that all std's came from contact with fluids. I'm sorry if that is the impression I gave in my answer. Certainly those that you mentioned do not necessarily involve fluids, but they can be transmitted by oral sex. Any oral sex can transmit whatever std is located in the area being stimulated.
He should be careful and he should be checked by a doctor.
Ed65

Oh, it's cool.
Yes, fluids definately do transmit all kinds of nasties, but I was just adding that even if there isn't fluid exchange he can still get infected with some things.