Talk to him about the burden he's putting on you.
Talk to him about what he's asking you to do. Point out how hard it's going to be to tank without proper armor, how much more likely you are to die, and point out that he's asking you to do this. Don't make him out to be the bad guy here though, just try to make him see how ineffective/likely to die your character is going to be with his restrictions.
Style up some heavy armor
If you have any art skills, draw up some heavy armor that fits the style. If you don't try to describe or google it. Show him that heavy armor can still fit his world just fine.
Talk to him about having magic armor.
Make it functionally identical to what ever equivalent you would normally have at that level. Or maybe craft the armor out of special/rare materials such as adamantite or mithral if they're available to the setting. This way you're still able to tank and his aesthetics are still held. This is a good compromise that should make both of you happy.
Talk to him about the burden he's putting on you.
Another thing you could try should the above fail, is talk to him about what he's asking you to do. Point out how hard it's going to be to tank without proper armor, how much more likely you are to die, and point out that he's asking you to do this. Don't make him out to be the bad guy here though, just try to make him see how ineffective/likely to die your character is going to be with his restrictions.
