You've already answered your own question. He isn't fully committed to the relationship as you would want him to be. My advice is to let him be, you cannot force him to involve you in his life simply because you are his girlfriend. Even marriages are of various degrees, some have deep connections while some marriages are superficial.
If this is all he can allow you in the relationship, then it is time to adjust your expectations accordingly. You should accept that he is not yet 100% sure that you're "the one'' and he is leaving his options open in case something better comes along. And that is ok, you cannot blame him for the way he feels. You can only respect it and allow yourself the same privileged. I suggest you do the same thing he is doing. Accept that he may not be the right one for you after all. Have fun with him when he is around, but don't involve him completely in your life, leave room for something better and more fulfilling to come along. Stop introducing him to your friends, and make sure you attend friend and family gatherings alone in case a better match for you shows up. There are no hard feelings here, you are only giving your boyfriend as much as he is giving you in the relationship. so all is fair. You will only feel cheated when you put in more in the relationship than he is willing to put in, or when you idealize your relationship when to him it's just a passing thing. It is important that you stay true to yourself first and foremost.