This is always hard. I’ve had a situation like this before to be completely honest. I was head over heels for this guy but distance was a huge thing. I say give it a go. What could be the worst thing? If you really care for him, you can’t just stop. Letting him know will point you in the right direction. Just be completely honest with him. If it’s meant to be, it’ll be. You’ll regret it more if you don’t do it than if you do and “what if’s” are always terrible.
I think that it’s inappropriate. If he has a girlfriend and is committed, he shouldn’t be texting his ex and ignoring his girlfriends texts. Something is obviously wrong. That is NOT right.