My comments are based on no particular expertise in this area. But, for what they are or are not worth:
How much time, effort, and expense went into developing your skills?
If it's per location, as he expands, so does your business. (And, described well, it can be present as a "win-win" business relationship. You're "invested" in his success. Though I don't mean giving him a break in return for anticipated future volume.)
P.S. I'm talking up front costs. No claw backs of those if/when locations close. Also, would a site license be transferable? That might depend on whether moving it requires your support.
noonespecial mentions support. I'll add liability (and limiting same). Make sure you define the terms.
I saw mention of LDAP or whatever. Sell him what he wants, in a timely fashion, now. Such things, if desired, can be future upgrades -- for a price. Even if you initiate: "You know, I'm continuing to work on the produce. Would you be interested in having this (let me show you how useful it is) feature?"
You may end up giving him breaks, and/or just being a decent business person. Just make sure those actions are on your terms and not coerced. You don't have to be a greedy bastard to want to maintain control over the situation.