While I can understand why some posters are jumping to the conclusion that your BF isn't that into you, I don't really think it's fair to say that without knowing any of the people involved. I was extremely busy with work, volunteer activities and other friends when I met my now DH. I certainly wouldn't have appreciated it if he decided that the limited time we spent together at first was because I wasn't into him. That wasn't the case. I wanted to spend more time with him, but found it difficult when I was working all the time and had committments to a charity organization, a sister who needed my help with her kids while her DH was deployed and my best friend's wedding to help plan.
So, assuming that he's just legitimately busy and genuinely forgetful (and assuming you've been together over 6 months), this is how I would handle your situation. First, you have to decide what your priorities are. Obviously, you want to spend more time with your BF and it's more time than he's currently giving you. So, is the time that you do spend together truly joyous? Does he act like he wants to be with you when you are together? Does he seem upset when you can't spend time together? Is the amount of time what's more important or how the time you have together is spent? In other words...what do you want more quality or quantity?
So if the time you do spend together meets your quality standards but you want more time together...either to progress the relationship or get to know him on a deeper level, then calmly and non-confrontationally let him know that's what you want. You don't want to approach him in an accusatory "you're not spending any time with me" manner, but more like "BF, it hurt my feelings the other night when you canceled our plans. I was really looking forward to spending that time together. Is there anything I can do to help you deconflict your schedule so that you don't have to cancel our dates?" Maybe one of the solutions is that you can hang out with him and his friends sometimes.
If, on the other hand, the problem is the time you spend together doesn't meet your quality standards, then you have a whole other issue. In this case, I would suggest thinking long and hard about what you're getting out of the relationship and if you think there's change on the horizon. I once broke up with a guy that I really liked. We spent a lot of time together, but all we did was hang out at his house, or mine, watching movies. He wasn't interested in going out much. When I thought about it, I realized our conversations weren't all that deep and I wasn't getting a lot of our relationship. I considered what it was about him that I liked and basically it boiled down to the fact that I was physically attracted to him and like the idea of him liking me. Balancing that against what I really wanted out of a serious relationship and he wasn't it.
At the end of the day, you need to decide if the amount of time you spend together is a real issue (and it seems like it is). I'd be prepared that he may not be able to find a way to incorporate you into his life any more than he has. Whether that's because his feelings aren't as strong as yours or it's just bad timing, is really irrelevant. If you're not getting what you need/want and if talking to him about it doesn't bring you any resolution, then you should be prepared to walk away and find someone that can meet your needs.
A friend once told me "The right guy at the wrong time is the wrong guy". And truer words could not have been spoken.