Did he take a tranquilizer? Offer him a cookie and a juice-box or a cup of coffee. I'm not going to suggest he pretend to have a good time ( there are already too many fake orgasms on porn sites), but perhaps vocalise a mmmm and/or wink if it feels good. Appreciate your er-uh girlfriend's effort, and make an effort that makes you both look like look like you're enjoying yourself and each other.