Answer:
It depends on what you mean by "dirty." You can ask a more or less ordinary question -- "How does your butt feel?" for example -- and use various obscene substitutes for "butt." Or you can ask him questions about actual sex acts (like masturbation, for example).
You can play "innocent" and ask him what certain four-letter words mean. You can ask him to describe how he masturbates (trust me, he does), in detail. You can play games: take turns using dirty words to describe the vagina -- and any other body part -- and see which of you knows more of them. You can play the same game with sex acts ("how many words can you think of for "intercourse"?)
You might even try some of these while both of you have your eyes closed, or by whispering them into each other's ear. Have fun!