Our MT is a pally but for the first 3 bosses in MH we decided to use our warrior tank. I was relatively new to the guild so I was assigned to tanking infernals on Anetheron. The other warr is another avoidance heavy tank with low threat.
One of the enhancement shammies told the MT that the warr's threat was low and he had to hold back too much. The MT told him to just go ahead and pull aggro to prove to the warr that his threat was too low.
If I were in your position I'd go ahead and just pull threat from him and tank whatever it is you guys are on. If you've told him how to improve and he hasn't taken your suggestions then it's his fault.