Oral sex is amazing, but I usually have to give out the same pointers.
1: It's not a sandwich; think of it more like an ice cream.
2: It is much easier for me to cum if I am confident that he is not bored and is prepared to go on as long as necessary. Paranoia about elusive orgasms only makes them more elusive.
3: If you don't like doing it, GTFO.
4: Don't be afraid of a little ass play, too...that's 3 things at once, which can be difficult to coordinate, but it's worth it.