Hey purplelotus, you'll find that your situation is very common in poly circles. As you can see in my signature line, I'm dating a married woman.
I would recommend reaching out to his wife in the hopes of building a cordial relationship where you can communicate about problems you might accidentally be causing each other, rather than solely using your shared guy as a go-between. This will also help build trust and compassion between you, which will make everything less scary, on both ends. Ideally, it could even develop into a strong friendship!
Your position is a difficult one, in that it brings up some big questions. Is he willing, potentially in the future, to commit to you at the same level as his commitment to his wife? If not, could you deal with it? Would you ever consider taking another partner to fulfill the needs he can't/won't if his marriage is the thing that will always come first for him?
There are some GREAT articles on dealing with these and other questions at www.xeromag.com
, check it out if you haven't already.
Best of luck!