I think his knee-jerk inclination would be to tell his past self to just… avoid the things that lead to him becoming a vampire? Like “Stay in school, don’t get involved in bloodplay with strangers.”, because he is pretty… fundamentally unhappy as a vampire? Like he doesn’t want to live forever, and have to drink blood, and be constantly lethargic, and abandon people who are like family to him out of the blue, and move around constantly to avoid being discovered for what he is. If you gave him the option, he’d want to like… grow old with his friends, get married, have kids and all of that’s kind of impossible for him now? But… with that said, he’s incredibly bad at making himself a priority and he’d worry a lot and feel really guilty about all the things that might not have happened had he not become a vampire, like… since he’s dating someone who’s also immortal it’d be a really big concern of his that by never becoming immortal himself in the first instance, he’d kind of consign them to being incredibly alone and isolated, and the thought of that would honestly make him feel way too guilty…
So in the end, he’d probably just tell his fourteen year old self that putting spaghetti in his mum’s rice steamer will not in fact, make it cook faster, it’s a terrible idea and he’s about to make a complete fucking fool of himself.