1) Do you have written specifications for what he/she will be building?
2) Do you have a written schedule for the building of this web site?
3) Do you have a written set of project deliveries, each of which is supposed to be complete by a specified date?
4) Do you have a written budget for this project?
5) Has the developer agreed to 1 - 4 (above)?
Treating your developer well is great, but without a plan in writing you are just going to drift away.
I worked for a web development firm and Gary's advice is spot on. Trusting someone is great but even kids and their mothers have miscommunication. So write it down and agree. The few items I'd add:
Those are to protect you as well as your developer.
Take a look at some of his work, and then verify he can program by having him build something in person. You may be able to get a feel for how productive he is going to be (Does he have to copy and paste all his code or can he work from scratch?). If he is your lone developer, is he going to need to know graphic design, a web coding language other than html, database design/interaction, security, etc. In other words, what additional help is he going to need.
Be careful when choosing your web developer. Get everything in writing and never assume.
Only pay them for work that has been completed not before.
What are you wanting the website to do? Just be informative? Generate Leads? etc...Your website should be built with your goals in mind.
Make sure the developer builds the website with your end results in mind. If your are trying to optimize the site from an SEO perspective then you want to make sure he/she has experience in building sites that are web/SEO friendly.