Ask the student to write a few sentences in English telling you about himself. Instruct the student to try to recall all as many of the rules of writing in English as he can for this exercise. When he is finished, have him read the sentences aloud and discuss some of the topics with him casually, without offering criticism. Collect the paper and use it to determine the level of competence the student already has, which will let you tailor your lessons more closely to his specific needs.
Provide the student with a short and simple English paragraph with all the spaces between words removed. Ask the student to add the spaces back in, which will help him review English vocabulary and spelling conventions. Repeat the exercise with different paragraphs, which you can choose to specifically target any problem areas of vocabulary recognition the student has. You can also remove punctuation or capitalization, which is something ESL students used to the writing conventions of their first languages may need to practice frequently.
Ask the student to respond in writing to a simple prompt that can be answered with a single sentence, such as "What did you have for dinner last night night?" Remind him to respond with a complete sentence, not a single word or phrase. For example, an appropriate answer to the previous question would be "I had roast beef for dinner last night," rather than just, "Roast beef."
Incorporate other writing exercises into your student's routine, to go along with those already used. Some of these should involve correcting commonly made spelling or grammar mistakes. Learning to notice these will make the student more likely to catch himself before he makes such an error. For example, provide the student with 10 sentences with mistakes in them -- such as, "The opposite of hot is old," and have the student correct the error.
Ask the student to keep a daily journal about his daily life or personal reflections, when you feel he has made significant progress toward writing better English. Each day's entry should be at least a paragraph long. Check this journal once a week and leave notes, some to correct writing errors and some to show the student you took the time to read his writing thoroughly. This latter element will help you establish or reinforce a personal connection with your student, which will make him more likely to respond positively to constructive criticism.