As an examination on grief, this is a great story. It shows how Oliver comes to terms with his brother's death in Afghanistan and how this shakes his entire life to the core. Everything he was working towards, all his plans for his future, are now called into question and he has to figure out who he is and what he wants without his big brother's light to shine the way.
As a romance... well, Oliver's 18, so I think that's certainly young enough to not call this GFY even though he does the whole "but I'm straight" thing for awhile, and of course Luca's the only guy ever he's seen in that way. But personality and sexuality tends to be less defined at that age, so it's not unexpected for Ollie to view himself as straight because why wouldn't he? Especially since his whole existence is being as much like his big brother as he can be. He gets over it pretty quickly and there's not a whole of him examining what it means to be gay/bi/whatever. I'm also not clear what Luca saw in Ollie. The whole thing is told from Ollie's POV and he's too self-involved with his grief and his woes to get a very clear picture on who Luca is apart from him.
This was all supposed to be taking place in the DADT era and early 2000s. There are some minor details that didn't ring quite true. There were also a few brief instances of casual homophobia when Ollie first learns Luca is gay, and one or two lines that could possibly be read as ableist, but ymmv.