He can relicense everything except the code from those patches, while keeping that code under the MIT license. He will be required to preserve the MIT copyright notice and license text, and if he wanted to initiate a copyright lawsuit he would have to exclude those bits from what he's suing over, but otherwise there's no problem. GPL-family licenses have a stronger need for "any later version" than permissive licenses, since the virality rule means you can't just combine code under different GPL licenses in the same program.

(Technically, the MIT license also grants the right to sublicense, so he actually could change the license, but only if the new license incorporated the MIT license's attribution requirement. Probably. The license is vague and I'm not a lawyer.)