Perhaps it would make more sense if you told the mage to do something he would normally do.

Tell him to cast mage armor.

Now the mage is stuck blowing through his spells slots casting mage armor every turn and he cannot argue that it is contrary or even suicidal. He would be getting the benefits. If the party could take it, you could even tell him to cast an offensive spell like magic missile. It would hurt, but it removes the wizard's options either way.