Even if it's a prologue you can add choices without changing the end.
Add a few false choices (maybe him learning the girl's favorite things and whatnot), and a few choices that loops back into the story later on.
(Like, he can try and run away from his mom, but is forced back. He tries not to eat and for a while it works, but then he has to do it to gain strength to protect someone he loves or something.)
There are many ways you can add choices without changing the end.
And even changing the end, you can do that in smaller ways that do not change the story itself.
(I admit I got frustrated with the lack of choices and skimmed the last part before I stuck it somewhere to read when I've cooled off my irritation. It's too good a story to bin, but I really am in a game-playing mood, not a novel reading mood. So I can't give exact suggestions there. I also don't know more than the demo for the main story.)
I guarantee I am not the only person who tend to get pissed off over a lack of choices.
So at the very least make sure you point out that the prequel have zero choices --or people will expect it, get disappointment, and possibly pissed off at it.
Feel free to ask for suggestions if you want them tho, to add choices I mean.
I'm a really lazy writer, but I've studied writing for some time, and written since I was young enough to send stories to a lego competition --probably as long as I've been able to write. lol
I am good with ideas, even if I'm bad at sitting down and actually writing them.