I want to get married but Islamically he has another wife?

As you may be familiar with the Islamic faith men are allowed to have up to 4 wives, being that they provide them with equal accommodations. Well living in the 21st century United States, that is not as highly regarded nor positively accepted by women of the Islamic faith. I never thought I would be in this position, but I met this gentlemen and after 5 months of a strong committed relationship we began discussing plans of marriage. However, before continuing our plans he revealed to me that he had been married for a year and a half in another country. Me being the women that I'am gave him the ultimatum of divorcing her and continue life with me. Four months later, of patiently waiting and agonizing arguments, he has not divorced the other lady and I'm left no solution but to leave. Besides this situation he and I have no communication issues, or problems within our relationship. I'm open to hearing any advice on this problem. Because truly I don't want to be "the other women" or be deceptive. Please give me advice.