What do I make of his flirty texts?

When someone ends a chat with you by being flirty, how do I know if it’s a sign that he is interested in me? A friend of mine is always doing this whenever we have a conversation on WhatsApp but doesn’t say anything when we meet. It’s as if he is a different person online and offline. I can’t bring up the topic because he never initiates anything, which is why I am confused. If he likes me, I wish he would just come out and say what’s on his mind. This just seems like a game, and I also sometimes wonder if I am reading too much into his messages. I don’t even know if this is how he communicates with everyone. Should I just ignore it and wait until he says something specific?
You have the option of waiting for a clearer sign or broaching the subject even if he doesn’t. However, it may make sense to gauge your own feelings for this person first. Do you want him to express interest in you? Are you interested in him? If this is something you would consider, nothing stops you from coming out and telling him how you feel. If his mixed messages are upsetting you, call him out on it and ask him to stop. This boils down to communication and being able to say what’s on your mind even if he hasn’t been able to yet. Approach it like an adult without worrying about potential awkwardness and you will be able to find a way out.

My ex-girlfriend recently messaged to say that she would consider getting back with me if things didn’t work out with her current boyfriend. I didn’t respond because I don’t know what to say. I miss her but I also don’t want to be her back-up plan. What do I tell her?
Tell her exactly how you feel. Being a back-up plan is not a nice feeling, and you need to ask yourself some questions about why this relationship ended.