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I had a friend in university who has a situation just like yours, that took her (now boyfriend) a long time to decide and commit.
Every guy is different, it may be family factors and other things, also stress and life in general. Try to be understanding to him and his situation. If you guys are so close now, there's no reason to resort to thinking that anything is wrong. Give him some time and he'll come along. It's good that you have told him how you feel. :)