First thing first, I'd suggest focusing on what you can do to minimize the impact to the student. For example, what can you do to lower the impact to his GPA: is your course an elective/required course? And has he passed the date to either drop or withdraw? Having him dropping the course, and focusing on language learning for another semester before (re)taking your course may be the best for him.
If he will have to stay in your course, then do an inventory of what you can supply. For example, how much time, man power (including TA), and resource that you can allocate to help this person. Here are some measures I have seen people doing:
- Allow the student to record your lecture.
- Make your lecture outline/script available. Or make some of the Q&A scheme available so that he can get a sense on the style.
- Provide a longer duration of time for tests/examinations. (Based on policy you or he may have to file for accommodation, check with your school.)
- Inform your TA about this, so that your TA can use simpler wording with him if necessary.
- Be somewhat conscious about your speech: minimize the use of i) local colloquiums, idioms, or jokes ii) somewhat indirect expressions like sarcasms.
Informally, you may also encourage the student to identify some textbooks that are in his language as a reference. It may also be prudent to first see how he did in the test this week first. He may be confusing the language understanding and the content understanding; or he could have some worrier personality that is aggravated by the looming test.
If it turned out to be really language barrier, programmatically you can also consider the followings:
- Check with the admission/student office of the program and see if they can connect the student with language service. Some of them may include paid lanuguage tutors, editors, etc.
- Connect the student with any kind of student life organization in the school, which may have resources on activities like workshops, peer-writing groups, social hours, etc.
- Contact the student's academic advisor, so that he/she may be able to suggest some relevant language courses for the student.
My advice is that, be professional and diligent in helping, but do not take this as your personal responsibility. Making your course understandable to every non-native speaker is not the job. Remember he i) knowingly applied for the institute, and ii) had likely produced relevant language competency during the application process, so the lion share of the responsibility falls onto him and the admission. Time to time, a student with great language test score may not come with an equally good listening or speaking; so, if you detect that the student is unlikely to make it, you should suggest him to withdraw for now and come back when he's more ready.