What to do?

There's this guy who's a seaman currently on board. He constantly updates me about what he's doing, which I appreciate. But when I checked his profile about four months ago, I saw someone had tagged him in posts, and I assumed she was his ex-girlfriend. I've asked him about her several times, but he keeps denying it and insists he's single.
I'm not sure if I should give him a chance or not. Should I accept him, or is this a red flag?