Well, you're not folding.
How much behind? If he's a frequent slow player - probably he has AT or over pair. I would assume I'm behind.

If he's not so weak as to fold AT or JJ/QQ here... then I guess you should just call. Aces are probably not outs for you. If you think he will fold a mere pair (even AA!) here then you can raise all in (or whatever the scariest amount to re-raise is)

You have 13 sure outs twice, although if he has a set then 7s might not be an out, and there might be redraws on the river to a boat.