Robynj,
Unless you have client/customer information or payment information on the site there really is nothing there that can be stolen or damaged beyond repair. I would be sure to back up the site’s database with an export first and if something goes wrong you can delete the entire install and re-install with the backup and be back in business in an hour.
Honestly the best defense you have from the contractor is the reviews you leave of his work. If a contractor did anything as irresponsible as violate the admin trust the blowback from your responses online would sink him.
I am currently at odds with a developer of a plug in who I was able to get back in line with a dozen or so bad reviews left in various places. He is now begging me to delete them but until he solves the problems he caused I’m not going to do anything to help him recover his online reputation.
You are not at the mercy of those you hire and the teaspoon of trust you are affording him is far less than the heaping table spoon of kick ass you can unleash should he give you a reason.
Another option is to have him access the admin of your site via your computer in a screen sharing session like Teamviewer. This way you can sit back and watch as he does his work, trust but supervise?
Totally up to you but I would create the admin for him, let him do the work, them delete the user. If anything comes of it you don’t like you do have recourse.