First of all, allow me to commend you for trying to give this young man some extra help.

The subject-verb agreement is very basic in English (unlike in other languages like German, Finnish, Russian or French). In English, you only need to differentiate for 3 pers sg and with -s added to the verb in the present tense, and that's it. As for the declension of adjectives, what do you mean exactly? Are you referring to adjectival form changes when expressing the comparative and/or superlative degree?

I could give some more concrete advice if you had mentioned the native language of the student as that would give me some idea of how to approach the "problem" when knowing a bit more of the structure of his native language. From the few details you have mentioned I conclude that his native language must be one where subject-verb agreement does nor exist; the same for word form changes of adjectives in case of comparative and superlative degrees. There are several such languages where such procedures of marking grammatical information do not exist - Chinese, Korean, Vietnamese, and Japanese. And there are other languages as well.

Do you think that your student could understand and would respond positively to a couple of sample sentences in English using these grammatical forms and others that do not? I would give him the examples first, do a bit of testing and practice, and at the end I would give him the rule.

Maybe, a contrastive approach - using equivalents from his native language along with the English equivalents - might also help.....

Is there a teacher speaking his native language you could perhaps find? This might help as well if the both of you could make a joint effort.

For the rest, learning a foreign language well is the same like playing football - the best way to become a good football player is practice, practice, practice. The same applies with respect to your student learning English.

Good luck in your efforts!