Connecting to SQL via IIS using integrated authentication

I have a web server on my domain that i want to use to connect to an SQL database.

Ideally i want my users to log on to the web server using integrated authentication, then the web server pass the same details on to the SQL server so i can use the built in audit tools.

I have enabled  Windows authentication and ASP.net Impersonate in IIS

my web.config file has the below

  <system.web>
    <compilation targetFramework="4.5.1" />
    <httpRuntime targetFramework="4.5.1" maxRequestLength="50000" /> <!-- 50mb -->
    <authentication mode="Windows"/>
  </system.web>

and

<connectionStrings>
 <add name="sitename" connectionString="Data Source=servername; Initial Catalog=databasename;Integrated Security=SSPI" providerName="System.Data.SqlClient" />
  </connectionStrings>

We are running on port 8081 as this is a back end API, i have added an SPN of setspn -s http/hostname:8081 hostname  (unsure if i had to do this i thought i would try it but didnt make a difference)

we are using web API to return the login name (user.identity.name) which returns fine on a page that just returns the information but to connect to the SQL server we are using entity framework which uses the above connection string.

We have a test page using the indentity name and it returns all the details fine on local host and remote host however for the entity framework we get this message {"Message":"Login failed for user 'NT AUTHORITY\\ANONYMOUS LOGON'."}

Is there an easy way to resolve this or an easier way to create a trusted SQL connection on an intranet enviornment.

Thanks in advance for any help provided.