If he's performing well and the guild is beating the content it's taking on, I can't imagine why they would even consider wanting to give their proven, trusted MT a demotion. No one but other tanks and maybe healers ever looks at a tank's stats, and you're saying he's got you beat in health anyway.
Put yourself in his shoes, and the guild's shoes.
On the other hand, if you're getting to raid and get upgrades and such, I don't see the problem. If you like the guild, then as you perform consistently you'll develop trust and get a good reputation; maybe y'all can work out a way to take turns on bosses, like my guild does (totally informally, btw).
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