Depends on if you are providing him a job. Note that contracts can be both good and bad.. for example, if you are dissatisfied with his work, there can be unnecessary legal litigation involved from both parties.
It's always important to have a contract associated with any service you offer online. Understanding the volatile service of forum posting, you may consider making this the first step before allowing the contracted poster to begin work. There are a few aspects of the contract you may want to consider in this instance:
1. Does the contract cover the duration of the assignment?
Do you outline the number of months you want to spend working with this poster or do you include your ability to terminate the contract at will? This information is vital to the contractor and yourself because it outlines how long you want to make this project. Before considering this consider if you want the poster to continue posting on your forum until the traffic reaches a certain amount, the poster reaches a specific number of posts or simply until you are able to access the overall financial standing of the site itself.
Recommendation - Set a post # limit at which you will do a quality check. If you advise your poster to send a message to your account every 20th post, this allows you to review their work and consider if you want to purchase an additional 20.
2. Do you have post specifications?
Consider if you want the contractor to post a a minimum or maximum amount of words per post. To avoid empty posts ( to include one word posts, thumbs up posts or simple posts with emotions), specify the number of words that are used as minimum for each post, the number of posts they are allowed to post each day and the guidelines at which you may have to include if you don't want them to post in a specific section. Clearly outlining your requirements allows both parties to come to a basis understanding of the position.
3. Have you discussed a payment option?
Do you intend to pay the contractor after a certain amount of posts, each post or after the overall project is complete? Determine your payment dates, merchants and instructions to request payment. Both parties should be fully aware of this information prior to the start of the project.
I recommend using a site like DocuSign for all contracts.