He is way to young to settle down and your age difference is a problem. He is not imature, he is just young and hasn't decided what he wants to do with his life yet and he shouldn't he has a lot of time to figure that out. You were fun while it lasted but he has a lot of "oats" to sow and he is young and want's to hang out with his friends and meet new people (girls). You need to move on also and maybe with someone a little more close to your age. I have also dated younger men but have always realized it was fun and would not last.