The answer in large part depends upon the nature of the relationship and how seriously you are dating. That is, has there been an agreement between the two of you that you would only date one another? The answer to that question would have a big bearing on what you should consider doing, but this is not only because he is seeing someone else in addition to you. It is a commentary on the fact that he may not take his agreements seriously, and this may extend to other areas as well. That is the more serious factor, and reflects on him as a person.
If there was no express agreement that you would see only each other, but you perhaps hoped that would be the case, the best course would be to calmly discuss the situation with him. Express your concern, express your understanding of the terms of the relationship, express any disappointment that you may have. Place no demands upon him. Then listen to him. Really listen. You may not hear all that you would ideally like to hear, but attempt to determine whether what you do hear in his words matches what you see in his overall behavior. Find out from him whether he wants an exclusive relationship. The answer has to be Yes in order for there to be one; but if his answer is No, you will have to accept that. Other kinds of mutually beneficial relationships can exist and you are cheating yourself, and him, by ruling them out.