Having trouble getting my "assistant" to put in work. How do I tell him to help out more without being rude?
Disclaimer lable: 100% organic locally grown opinion

I think there are a few aspects you need to reflect on.

First, it's fine for different people to create a joint effort with different motives. As long as the motives move the main goal forward. In addition, just because he wanted to publish a paper does not imply that he does not value learning. If his saying made you assume that he is just on for a free ride, I think that can be too premature a conclusion.

Second, learn how to manage people. Don't expect them just to fit in perfectly with your schedule and work style. Set up ground rules, lay out tasks, and negotiate on the ways of reporting. A very good feature about this case is you know his Achilles' tendon (or hanging carrot if you're a glass half-full kind of person): Name on the publication. Lay out all the works or tasks or learning for him and demand him to complete them on an agreed time or his name will not be listed as an author, but in the acknowledgement. Be civil about it and listen to what he says.

Third, sort out your own conflict. First:


  How do I tell him to help out more without being rude?


And then:


  but at the same time, I also struggle in giving him work to do. I do
  not want for him to take credit for something I did


Actually do you need an assistant or not? If yes, then make sure the terms are clear and he followed through. If he did, and ended up in the paper, he will eventually take some credits of yours AND you will also take some credits of his. This is what collaboration entails, at the end we have a general idea who did what, but it's inevitable to have some shared credit. If you're completely possessive about your work, and you think that he is taking advantage of you, then you should consider if you need an assistant or if you should decline this responsibility assigned by your supervisor.

Lastly, strengthen yourself up. Listen to this:


  I am afraid that because I may appear weak to him, this will most
  likely happen.


What about you that is weak? Why do you just label yourself weak from the get go? Personality? Physique? French? The only thing you need to be stronger than him in this case is your knowledge in this project. All others are side matters and you should not place yourself in the victim seat. An effective manager does not have to have strong personality, he/she just need to be fair, just, and remain clear and open in communication.

Lastly lastly, you should also talk to your supervisor for advice and tips. Work on it to create a happier collaboration experience.