Marykate:
I am a native UK English speaker, who teaches English. I usually teach TESOL, but I also teach English to native English speakers, children and adults.
Very generally speaking, large numbers of English people of my acquaintance speak English ungrammatically. Their ability to explain grammar, and to use grammar correctly, especially written grammar, is often almost non-existent.
I agree with all of the excellent suggestions that you have been given by the other teachers.
In addition, if your student is as good as you say, at both English and Spanish, put him to the test!
Obtain four books, (preferably Children�s Stories, suitable for your class), with identical text, two in English, two in Spanish. (A good alternative, would be to buy ONE dual-language book for you, and photocopy appropriate pages from it for your student, for each lesson.)
In class, you let him use one book, for that lesson, in Spanish. You have two books, with identical text, in English and Spanish.
At the end of the lesson, he DOES NOT keep the book, he RETURNS it to you.
You have the ADVANTAGE over him! You have previously read both copies of both texts, and now have a good knowledge of the grammar, vocabulary, etc. IN BOTH LANGUAGES!
He reads the first sentence in Spanish ... you correct his pronunciation where necessary, ask questions about Spanish grammar, etc.
If you wish, you read your copy to the class, aloud, in Spanish.
Now, you ask him to translate the sentence into perfect English ... and ask questions about English grammar, spellings, pronunciation, etc. (because you have the English text in front of you!)
If you wish, you read your copy to the class, aloud, in English.
You invite the other students to ask questions of him, etc.
Continue with the next sentence.
If you wish, pass HIS book to the next student to have a try.
At the next lesson:
He is given one book, in English. You have two books, in Spanish and in English.
Now, he reads in English and must translate into perfect Spanish!
You correct his Spanish, and so on.
You are learning, he is learning, and the rest of the class are learning.
I hope that this helps you.
Les Douglas.