Language skills of your child at this stage stand on three pillars- Vocabulary, grammar and pronunciation. So paying proper attention to grammar is a must. It is not expected from a five year old child to come up with impeccable grammar concepts. But, what as parents you need to look at is how well he can speak or write a grammatically correct language. Be it English or any other language, without grammar the sentence and the composition will not have much meaning.
What you need to know
While it is pointless to expect a child to memorize detailed grammar rules, it is also important on the other hand to make him aware about what grammar is all about. At this age, kids don't understand rules, but they do understand what sounds correct. For example, a child can easily tell you which sentence is correct: “Harry going to school” or “Harry is going to school." Your child might not be able to reason it out but he will tell you which sentence sounds appropriate. This is because the child learns everything from what he hears.
What you can do
Thus, to hone your child’s grammar skills, ensure that you speak grammatically correct language with him. Correct where he is going wrong without referencing rules. Tell him what grammar is in a simple way and also enlighten him why it is important to speak a language that is grammatically correct. These small steps will ensure that the child has a concept of language right from an early age, and this will stay with him all through his life.