Another thing to think on is to try to write stories that you'll both like. You say you're not into inflation yourself well then add what you like into the story as well. I know I find it harder to write about something I don't really enjoy so in that case I'll add in some stuff that I do like. That makes the process easier and more enjoyable to me.
Also it probably wouldn't hurt for you to have a look on the internet for other expansion stories. You say you use the same general idea well have a look around see what else is out there. Reading other stories will give you ideas of how many different ways inflation can be done.
And Alorok is definitely right arousal and expansion are generally tied. The bigger the person inflating gets the more turned on and hot they become. Now mind you this isn't always the case but for the most part it is and I assume this is what you're going for.
But again I would say the biggest thing is don't just write it for the sake of writing it. It's great that you want to do something nice for your boyfriend. Just don't be afraid to make it so you like it as well. It'll make writing it and reading it more fun.
Well there are hundreds of things that can be said for writing something that is romantic and/or erotic, writing an inflation piece is its own challenge.
If the story is meant to be straight out erotic. Then I recommend comparing the inflation to sexual arousal. As in the longer it goes on the more hot and bothered both people become. As well as that sense of pressure and trying to hold physical passion in, or just releasing it. Try to compare size and descriptions with normal arousal.
If you're looking into more romance, then having unique unrealistic characters really helps. But familiar, try to think of reasons why a character would be looking for love in that way. Would either lover feel that inflation is odd? Or is it simply a way of expressing physical love in an unusual way? Or totally normal?
As for scenarios, there are several scenarios that have become tropes in themselves. But again that depends on if you're writing a full story or just a piece of romantic erotica. Most normal romance cliche's can be used as a basis for inflation love stories.
So... yeah. My advice is to focus on writing a romantic/sexual piece and compare the inflation to physical arousal. Yeah... that's all I can think of saying.