Past tense anyone?

How much  should a 5 year old know about past tense? Not much.

3 days ago, I did some revision with a  standard 4 student while Reese quietly played at a corner. I was testing my student on past tenses and half the time he couldn't answer. Reese was listening and he answered for him. 

After my class, out of curiosity, I took a standard 6 (local English) grammar book and tested Reese on past tense and I was a little surprised that he got almost all right. 

 Reese is always in the classroom during my lessons. He has his own space with a drawing table, books and toys. Lately, whenever I do comprehension with my students, he will quietly listen. After the class he will hurried to me and ask questions about the stories and also words he didn't understand. Then he will try to use the new words he learned in his daily speech. He speaks grammatically correct English 95% of the time. He is into writing sentences and requires very little help from me. He is able to spell quite well too (own initiative). I guess whatever I am doing with him for Language Arts is working. He has a flair for the language.