Assess the student's competency level. Several websites offer cursory or practice tests which require students to display simple knowledge of subject verb agreement. Starting here will help you understand where the student is and what needs to be focused on.
Review the mistakes from the student's test. Look for patterns in error. A student may have particular trouble with irregular verbs, such as "has," with matching pronouns with verbs or with pairing multiple subjects with a single verb. Zeroing in on the areas which pose the greatest difficulty allow you to provide more focused help.
Use practice sheets which focus on the areas of greatest weakness to strengthen the student. If the student seems completely confused, begin at the beginning. Explain to them that verbs are the action words in a sentence and that subjects are the persons, animals or things conducting the verb.
Get the student up and moving. Dictate sentences to them where they are required to stand when they hear the noun and complete the action when they hear the action. To make it easier on them, start by using their name in a simple sentence before graduating to more complex ones. Students can pretend to become other people or even animals. For example, the sentence, "David runs fast" would have the third grader, David, stand at the sound of his name and run at the sound of run. Pause after each word to ensure the student understands the components.
Ask questions when using worksheets. Instead of just looking for the selection of the right answer, ask students why an answer is the right one. An ability to explain the rationale behind a selection displays mastery of the subject. For example, if a student can say that the correct answer is "ran" not "runs" because the subject, Susie, conducted the action yesterday, then he understands the irregular verb ran and its past tense conjugation.
Encourage the student. Conjugating verbs is a difficult task and one that gives not only children, but non-native speakers and even some adults, trouble, from time to time. Let them know you have confidence in his ability to learn the material.