I think you should text him back, but don't get drawn into a text conversation. Don't arrange to meet him either, except maybe in a 'see you at the gym sometime' sort of way. Then when you do see him at the gym, tell him that you think he's nice but that you're not interested in dating him. Tell him about a recent split with an ex or something if that's true, but be nice to him without flirting. Hmmm. That's not very useful. I tend to think that being truthful in this sort of situation is much better than an elaborate lie, because if you get caught out you'll look kind of silly making up stuff to discourage a guy you haven't even been out with yet. That said, people being over-keen is quite annoying, as though they're just setting themselves up to be hurt. So if he seems like a genuinely nice person, be gentle, but make it clear that you don't want to go out with him.