There are many reasons why your student produce the speech the way he do.
(1) ignorance of english rhythm, meaning that he is not aware of the metre of spoken english
(2) if he is aware of the metre, he gotta know how to reduce vowels in unstressed positions.
(3) If he mastered the second step, he may still have problems in stressing english short vowels: here, you need to train your student on the power of consonants prior to the stressed short vowels. Releasing stops slowly, holding a fricative longer, etc.