He is testing your server for SQL Injection, specifically this is a robust test that will work even if its Blind SQL Injection. Blind SQL Injection is when an attacker is able to execute SQL however, there isn’t a viewable response. If the http request takes at least 15 seconds the attacker will know that he can execute SQL, and that your running MS-SQL. After this attack he will follow it up with a xp_cmpdshell() to infect your server.