If they have been talking about marriage, then now would be the time to talk about what their sex life may look life - in a safe place. Perhaps with a marriage mentor couple or at Starbucks. If marriage really has not been regularly discussed then I would recommend he wait until the ring is on her finger.
This young man may be very anxious to be talking about these things, but as virgins he needs to be cautious with his bride-to-be as he may scare the living daylights out of her. She may be thinking touch and kiss and slow and careful and he's thinking frequency, oral sex, multiple positions and hanging updside down already.

It takes time and sometimes constraint on the husband's part until they can get used to each other as a newly married couple. Otherwise, he could come across as a selfish lover instead of a giving lover to her.
There is a book by Dr. Kevin Lehman called "Sheet Music" where the first 1/4 is for pre-marriage and they say save the last 3/4 for marriage.
A great marriage gift for a couple like this would be "Red Hot Monogomy" by Bill and Pam Farrel.
"Before You Say I Do" by H. Norman Wright has an excellent section on sexuality and "Saving Your Marriage Before It Starts: Seven Questions to Ask Before (and After) You Marry" by Les and Leslie Parrott is wonderful too.