I'd read a few scenarios into this.
Ask him what he wants and discuss what you want/expect from this.
Guys (me) can see a FFM in one of two ways.
1: There's now 2 times the girl for me to pleasure myself on. (I'm not a big-headed enough to think I could please more than one.
)
2: There's another girl and my wife is getting off on her and I'm happy to watch, participating if necessary.
Some questions to ask yourself:
Do you want this for you or him?
Does he want this for you or him?
(Asked already) Are you okay if the girl wants him?
Is he okay with that too?
Are you okay if he becomes the subject of attention?
What if the girl wants you both to focus on her?
Maybe watch some porn and point out which 3-somes scenes appeal to you and put them on your 'wish list' for the event?