I'm with bluemagic. Ask the teacher to explain. I am hoping that "D" means something like "developing" and the other "grades" are like I for "integrating" and "M" for mastery." We had something like that last year for K. Our district has switched to Common Core for this year, though, so maybe I'm in for a nasty surprise.

Best of luck. I would NEVER tell your son about this "D" if in fact it is an ABCDF grade. First grade is not the place for that. Period.