Well you can let him know that usually you are only available in the evenings between certain times, that way you can text him back between those times and after or before that you don't have to respond if you don't want to because you have let him know you won't be available.
It's good that he is excited about it but some guys can come on a little too strong and it can be rather annoying.
I had this one guy start off as a nice enthusiastic texter and within a few days he was texting me at 2am telling me that he loved me!! Yes love after two days. Well I told him not to call me back and that it was going way too fast. He didn't stop and became more obsessed. I'd only been out with him once at that point so I don't even know how he ended up parked outside of my house watching me through my bedroom window (I use to live with my mom at this point I was in school still and had not yet met the guy who I would marry) but my sister and I were so scared of him! When I saw him out there I had to call the cops...and I wish I could say that it was the only time something like that has happened.
Anyway I'm not trying to scare you but just set your own limits and rules.