Basically it's a cultural thing. Muslims marry people deemed 'suitable' by their family - basically a Muslim virgin known to the family. One of my friend's brother (Iraqi) is currently in the USA having a wife selected for him. He, like many muslims was allowed to sow his 'oats' with western women prior to the marriage (I'm not saying this is the case with your circumstances but it is fairly commonplace). Interestingly his brother is marrying an English catholic but this caused problems - he was basically kicked out of the family and has given up practising Islam.
Again I don't know your circumstances (and I really sympathise with you because it must be devastating) but Muslims tend to have a loyalty to their family first and foremost. You may be able to prevent him marrying someone else by converting to Islam and therefore making yourself more 'suitable' in the eyes of his family, but this is probably unfair on you.
BTW it's not just a Muslim thing, many Hindus and Buddhists have arranged marriages.
If you want my advice you should be frank with him about how upset this is making you and trying to get an idea of his plans. If he is going to go along with getting an arranged marriage it may be better to consider your long-term happiness and get out now. However, if he loves you as much as you love him then he may go against his family. Either way it's going to be difficult for all parties.
You could probably tell right now where the future lies. If he has introduced you to his family already and they have accepted you then I would say it may work out well for both of you. If this isn't the case then I'm really sorry to say it could turn out badly.
Either way, I wish you luck and I hope whatever happens is best for you long term. I had a similar situation with my wife who is ethnically Chinese. It took me ages to get accepted by her family but now I am like their son. The difference I fear between my case and yours is that religion wasn't an issue - it was a cultural thing. Unfortunately Islam is fairly 'non-negotiable' in this respect
My best advice is to think about what you really want, and speak to him about it and demand an honest answer. Based on that answer you can make the decision that is best for YOU. But you really must consider your happiness and your emotional needs - how are his family going to react. Try and speak to them (with him) about it. If this isn't acceptable to him and his family, then hard as it may be, I think he doesn't have your best interests at heart.
Either way, Good Luck, and strength to get through whatever comes your way
I used to think the brain is the most fascinating part of my body. But, hey, who is telling me that?