What does it mean when he says he misses me?

[Ok...]

He's doing what a lot of guys do: Fishing.
He is with another girl, and so he sends these "miss you"s to all these other girls, to see which one will make contact with him. Whichever one goes for the "bait", then that's the one he will lead to believe that he wants to be with her again. Until he gets what he wants, then he will move on to the next better thing. Guys do this ALL the time. ALL THE TIME. Don't believe me? Consider this:

If he missed you TRULY THAT MUCH, he'd have broken up with the current girl, and made an EFFORT, by PURSUING you, to get you back in his life regularly. What he sent you was a crumb... I personally wouldn't respond. You have other guys to meet, that won't send you mixed messages like that... they would actually want to be with you in real time, and make time to do that. To make it happen... instead of be with another girl. Good luck.