I think the coach informed Jaekyung about Dan and his grandma's situation. Just a guess

What happened to Dan grandma what did I miss?

She's like... really really sick... Dan misses her so much... author showed the past story of Dan with his grandma... and uhh... well these things happen in a manga before someone dies. I am not sure. It's a guess.
And Dan told him he would stay with Jaekyung till his Grandma is alive. So maybe ...
But as i said .. idk