[MiddleEndian]

As mentioned in other comments, another wormhole or a more direct path to cut him off.

[chrisfosterelli]

Actually, of interest Wikipedia seems to say that you wouldn't have to leave Earth at all (just bring the other half of the wormhole back), so I think you're right that it doesn't solve the grandfather paradox: https://en.wikipedia.org/wiki/Wormhole#Time_travel

[MiddleEndian]

Cool, seems we've reached the bottom of this one!