Threesomes, as a general rule, tend to be very destructive to relationships. I’m sure there are some relationships out there that benefit from it, but only because both partners are really into it and are committed to all that is required to keep a relationship going with that kind of added tension. Honestly, I wouldn’t interpret your boyfriend’s request as him being bored with you (it is a fantasy of most men after-all), but in the same breath, I don’t think it’s a good idea seeing as how you don’t seem too excited about the idea.
Perhaps something as simple as adding a large mirror to the bedroom, could do a lot to spice up the sex in your relationship without having to bring another person into the bedroom. The mirror could help him satisfy any voyeuristic desires he might have, while possibly adding to the intimacy between you two. Just a thought.
Oh, and for the record, I personally would much rather have a threesome with two women than one and a mirror, but hey, sometimes you’ve got to take what you can get :P