correct verbs

John was obese. His classmates nicknamed him 'Fatty'. He was very sad and confided in his form teacher. She advised him to eat less and exercise more.

Two months later, he was no longer fat. In fact, his classmates were surprised to see that he is/was slim.

John is glad that he has/had listened to his teacher.

1. Should Fatty be enclosed by quote marks? It can be, but it doesn't have to be.
2. Should I use 'is' or 'was'?Was. The other tenses in the sentence are in the past tense. This one should be too.

3. Should I use 'has' or 'had'?Had, but it would also be ok without either had or has. John is glad that he listened to his teacher.
Many thanks in advance