A very promising coming-of-age story. It was fun and endearing, so very well done.
In terms of feedback, I agree with some others comments. Some facial expressions were kinda weird, but they weren't as many to be a problem. But, let's be honest, we people something make weird faces, so, ironically, it feels natural, organic, more so, since you're portraing an awkard young adult discovering his sexuality and himself. So, I think that, as long as doesn't affect any key moments of the story, is not that big deal.
Some sentences and expressions need some corrections, because can slow a little the pacing of the story when you are reading it. But, again, since is not your first language (mine neither, so take this part of the review with a grain of salt because I'm on the same page like you, haha), the overall work is very well executed. So, a minor fix here and there and thats all.
The characters are charismatic and sexy, and the dialogues are good. I would like, anyway, read a little bit more to have a bigger picture, but so far, so good.
The context, the enviroment of the story, this fancy house in the middle of nowhere during holidays, is a very good way to create an ambiente of closeness and intimacy (and also, for crazy things to happen, haha). I really like it (I want a house like that, to be honest)
In terms of ideas, I think it would be nice to read about Silvia, the aunt. Not now, anyway, but in the future. I know its a gay game, and the expectations can be others, but you're telling a story, so that's the reason why I think (at least a little) it would be nice to have content of her and her story.
But, if we talk about the spicy stuff, I have 2 primary ideas:
1- Since they sleep together, a lot things can happen overnight (like waking up with one hugging the other; for more fun and akward situations, Fabien should be the one doing this)
2- The family has a gym, so I think a muscle worship scene would be just fine. Maybe the Uncle can be the one encouraging Mathew to touching him and so (he's has such a big lewd aura, haha)
So, that's all. I hope my review help you in any way. Hugs and good luck with this amazing project.