Is it ok to break up with someone over email under these circumstances?

I started seeing this man in January. We saw each other no more than once a week, although we were sleeping together. I never really anticipated it going anywhere, but we never talked about the future, exclusivity, etc.

He left in May to travel for 3 months. I had somewhat expected things to fizzle out while he was gone, but he has been emailing me pretty consistently and telling me about his trip and sending me photographs and asking me to send pictures of myself.

For a variety of reasons, I’m feeling pretty lukewarm about him, and am not especially interested in continuing to sleep together when he gets back, although I guess I haven’t 100% decided that I don’t want to. However, he keeps saying flirty things and telling me how much he’s looking forward to coming back and seeing me.

What should I do?