Just tell him or the teacher that you can't read his work. Maybe the teacher can't either?

Write, "Je ne comprends pas. Je suis un dicque." If he complains that he doesn't know what a "dicque" is, you reply, "Now you know how I feel!"

Honestly I don't think it makes you look like a dick. If you put it in as friendly of a way as possible, it can be a good indicator for that student to go and visit his ESL adviser/teacher (most schools with foreign students have one) and get help with effective use of English. I'm sure he really wants to improve.

I had a similar situation in my masters program. One of the foreign students had a much better grasp of accounting than of English grammar (to be fair accounting makes sense) but we'd have to swap and do peer review. I didn't mind helping on small assignments but I would regularly spend more time fixing or pointing out problems with his English than with his accounting. I talked to my professor and he advised the guy to run all of his papers past the library's international help desk first.

You should explain it to the teacher

I know exactly how you feel. There was a kid from Kuwait in my engineering program and he wanted to write the report because the portion of the project we were assigned wasn't his specialty. Let's just say English wasn't either. I had to rewrite the report he gave me to check (my grade depended on this report too) as it was so broken I barely understood it and I knew the context and subject he was writing about. After we turned the report in he got upset at me for changing his report so much. It was bad enough I wouldn't let him write the next segment (I informed him as politely as I could) and he had a meltdown in front of the class about wanting to learn English and I wasn't letting him. We were all graduating with our bachelors in 3 weeks.