I'd say ------

1.) Talk it through first (preferably while cuddling in bed) so you both know what is expected/permitted

2.) Make triply sure you are both feeling into the idea on the night

3.) Third party is there as your sex toy, so once you've all had your fun he/she should leave (Come, Cum and Go) because you and your SO will have some serious glowing to do together. Don't be surprised if you spend the rest of the night getting off together and feeling VERY NAUGHTY.

If he's doing the suggesting at this stage, I would take the chance to yes to an MMF first, and a FFM next. Fair's fair, after all. If he goes for this, you can be sure that you are the first thing in his mind......