You don’t owe anyone romantic or personal interest just because they’re helping your career. The important move here is to shut down the flirting firmly, but in a way that doesn’t create unnecessary drama. For example, if he sends you a text that feels too personal, you don’t need to get drawn into it, just reply with something work-focused, like shifting straight to schedules. That way you’re not giving him fuel to keep going.
If it gets to the point where you feel you need to address it directly, keep it respectful but clear: something like, “I really value how you’ve been steering my career, and I want to keep our relationship focused on that. That’s where I feel comfortable.” You’re reminding him that you respect his work while drawing the line.
And in the background, it’s smart to quietly start building other professional connections. That way, if he doesn’t take it well and tries to make things difficult, you’ve got options and you’re not trapped. A good manager should want what’s best for you, and if he can’t respect boundaries, then ultimately he’s not the right person to be guiding your career anyway.