Seventh Grade: Teach your student to apply his grammar learning to writing.
Hopefully, this has been happening even earlier than seventh grade because seeing the “why’s” of learning something (“I need to learn prepositions so that I can spot prepositional phrases so that I can be sure that I have accurate subject-verb agreement” or “I need to learn how to punctuate double and triple adjectives so that I can write with them in my descriptive paper”) is extremely motivating to students.
One of my mantras is “Phonics is for reading and spelling. Grammar is for speaking and writing.” I try to teach my students from the beginning that every grammar concept that they learn is applicable (and needed) for writing.
One way you can do this is to create an ongoing checklist of sorts of each pertinent grammar skill (as he learns that concept in his grammar book) that your student can use in his writing. For example, after your student learns prepositional phrase openers, add this to his checklist for writing that week: “Add one prepositional phrase opener to your essay.”
Note: See one of my Checklist Challenges in my samples of Meaningful Composition here. This will give you an idea of how to create your own checklist—or you can print mine off and use it with your students.