We like someone either due to his/ her nature, habits, smartness, intelligence etc. or attraction for fascination. Since we do not know your age it is difficult to guess about it. If you carefully observe the people you like, you will find that everyone has one or more of the above qualities and you like everyone because of his/ her qualities. Now, as you said, you like him most, he must have more than one qualities mentioned above and if it is not so then certainly it is just a fascination. In teenage, children easily get attracted/ fascinated towards other person because they have not got the chance to meet many people.
Today you are liking him but when you will meet other people in the coming days, who have more qualities than him, you will get attracted towards them. Since he is already in relationship with other you must treat him just as a friend and rest assured there is no dearth of good people on this earth. Other day you will find some one much better than him.