My oldest, who is in 7th grade, has never had much formal grammar. He decided he wanted to take a high school level Latin class this year, and his lack of grammar was a concern to me, particularly because I have a minor in Latin and know how challenging it can be.
What I’ve found is that the Latin has helped him with his grammar – actually he is learning Latin and grammar in that class. It hasn’t slowed him down at all. He’s almost done with the class and is maintaining a 95% average.
So, for us, his lack of grammar wasn’t an obstacle.