Hey there! Welcome to PC!
My first question is how long have the two of you been officially dating? This isn't an uber important question, but it will help me understand your relationship a little bit.
I also wanted to mention the fact that you are still quite young (I know, this coming from someone only a few years older than you isn't going to sound like great advice). When my boyfriend and I started dating around that age, we both still had a lot of maturing to do. I would purposely pick fights over things I knew we didn't agree on as a way of saying "see, we're not compatible! We're never going to last!" It was pretty dumb, but for some reason, I really felt like I had to stick it to him, that his opinions were wrong, that we weren't right together. Like you, I'd often get really upset with his answers to questions I'd ask. But after breaking up for 6 months and getting a little older and a little more mature... Those strong opinions we had about things began to change. I guess I just wanted to let you know that, while you might not think so now, opinions on having children and how to raise them... they're likely to change a little, for the both of you. There's no need to worry about these things now -- you're both still so young, enjoy life while you can!
The next thing I wanted to mention is that a relationship between a son and his mother are... well... They're tricky. I can't quite explain it (not being in one myself, I'd probably be quite useless if I tried to explain them!). But you can't be too hard on your boyfriend because of how he interacts with his mother. He still lives with her, and still has to follow her rules to some degree (of legal age or not, he's still some what dependent on her). Also, it's even trickier cause she's dealing with cancer. He just lost his dad not that long ago, and it appears he's going to be losing his mother sooner than most 18 year olds. While you are still young, and not married, try not to let their relationship affect your relationship with him. And, another tid bit I learned from my boyfriend... His mother, while I won't go as far as saying she hates me, she is definitely less than thrilled with the choice he made in dating me. But I always thought my boyfriend was like yours, never stood up for me or anything. I was actually wrong, he has stood up to her for me, he just never felt the need to tell me this. Even though his mom didn't change that much (which is why I never noticed he had actually said anything to her), it was still reassuring to know that he did stand up for me, on his own free will, in his own way. I guess, I just don't want you to judge your boyfriend too harshly, especially when it comes to his mother. He may be standing up for you in little ways that you don't notice. He is probably also struggling to keep some sort of peace and balance. You also don't want to make him feel like he has to choose between you or his mom, because that's not necessarily fair.
The last thing I want to mention is the whole being in a rut business in bed. My best advice here is to talk to him. He probably doesn't understand what you mean by "be spontaneous." He probably needs some more direction. He is doing his best to keep you interested. I feel that you may have put intimacy on a little bit of a pedestal. Perhaps just try to open up the lines of communication a little more; help him understand exactly what you need/are looking for.
I hope I was able to help a little bit. I know when I post, I often have a tendency to ramble a little, so I apologize if this was long winded, confusing, or just plain useless.
Best wishes and I hope everything works out for you!

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Ro