In his case I think he just fell in love with his saviour? y'know the 'They're the only person who cares about me' typa trope. I doubt he would've been interested in his sexuality or love because he was barely hanging on for dear life but was also being abused and poisoned by the old hag. Honestly any person who had or will enter his life and show that they care for him, wish for him to get better, help him when he is in need of it, provide a positive atmosphere, and most importantly communicate with him....there is no doubt that he would fall for that very person who is a symbol of life and light to him.
Also I understand that not everyone is fond of the 'this was bl but he became straight' tropes however, I don't mind them UNLESS they are blatantly showing or hinting some sort of prejudice or dislike towards bl. That aside tho, these types of stories will do fine with either genders being mc because the stories that they usually transmigrate into are ones where the ml/fl just need one person to be on their side and give them the love and attention they need.
I'm not sure if there is a sexuality specific for his situation, correct me if I am wrong but I assume this means he is pan?
These typa stories are confusing with the whole “this used to be a BL but…now the ML is straight cuz i (the protagonist) appeared!” Like bruh are these authors allergic to a little sprinkle of zest or somethin?