To back up a step and decide WHY he wants to have this experience. Bringing another person into the bedroom is a loaded topic that not even freewheeling poly folk like myself take lightly. The time to approach this topic is from the very beginning of the relationship it's not a tool for spicing things up unless BOTH parties are on board for it.

Now he would be wise to do things like read forums where the topic is discussed reasonably...like here or find "experts" who can tell him the downsides as well as the upsides to the whole proceedure.

Watching some porn with his girlfriend to break the ice...and then asking in a loving manner how she would feel about trying a threesome would be my advice. LISTEN to her words and be willing to drop the whole idea if she is not willing to discuss it...period. Coersion is the quickest way to lose a relationship and demoralize your partner. Telling your partner about fantasies is great and everyone should be willing to do this but trying to force a fantasy on a partner is horrible and NO ONE just decides they like it after the fact.

Still with a gentle touch and a loving manner many a partner has decided that they would love to try a new experience and been very happy with the outcome. There is no easy answer with this question.