Maybe he chose to be with her because it comes easier to him. I had the same experience with one guy. He would always pick other girls as girlfriends but he always spent hours talking to me. Later in life he confessed to me that there was a time when he had a crush on me but never acted on it.....
You know what I'll suggest? Keep in touch with the dude. Talk to him, invite him to do interesting things with you (bowling, pool, hiking...think of something). Show respect for his relationships but also make him SEE you. In time his -probably- immature boyish brain will shape up (HOPEFULLY) and maybe this will lead him to look for something more....(remember though...try not to waste too much time on this. if you see things develop then go with it...but if nothing happens, then...I guess it was just worth a try!)