How did he connect the rocket to the planet? If it's inside the atmosphere, then your rocket exhaust has to fight its way through the atmosphere, which would transfer most (all?) of its momentum back to the planet/atmosphere, and make it fall back down again. If it's outside, you have to transport the fuel up to it, and then (assuming the rocket is in front of the direction you want to force it in) fire the rockets sideways enough for their exhaust to again miss the atmosphere (so that, relative to the planet, it looks like it's hovering, but it's really pushing the exhaust somewhat backwards), or you've again just transferred the momentum back to the planet. Or if you want to put the rocket behind the planet, you need a rigid connection capable of taking the compression between the rocket and the planet.
And then you have to work out the force needed to make any kind of dent in the forces on the planet, compared with the others it encounters (how did he get Uranus/Neptune past Jupiter on the way in? This presumably took thousands or millions of years, so did he move it out of the plane of the solar system? Can you do that and keep it far enough away from Jupiter at all times that it would get thrown off course by Jupiter's gravity?)
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