Your child has now learnt to spell different words and small sentences without any difficulty. He can converse with you on various issues by making sentences on his own.
However, there might be times when he goes wrong with the tenses. It is your responsibility as a teacher to correct the child. You have to make the kid familiar about past, present and future and based on that make him learn how to frame sentences.
What you can do
The basic step is to converse with the kid. Make him talk about what he did or what he is doing at present. This would help him to differentiate between past and present tense. Let the child make mistakes because then only you get a chance to teach him the exact words.
Now you can also start to teach the kid about different words and their correct form when used in a different tense. This knowledge is very important and would help the child improve his speech.