The format/visuals/teaching methods are different at school... than what he is used to... doing at home and with you.
Thus, perhaps, at school, he is thrown off, by it.
Versus at home, you have used those same....learning tools with him, repeatedly and routinely, and he has "mastered" that. At home.
Flash cards, are pretty much memorization.
But at school, they learn things, in a variety of ways. Not just in one, way. Many ways. Even when just learning numbers or simple addition, they do not teach it in only 1 way. They teach it in various manners using different tools.
He is used to, the routine at home and has this memorized.
Many kids, learn routines very well. When it is the same thing all the time. It is repetition.
But not at school.
The manner in which he is learning it at school, is different.
At my kids' school, they do not teach reading/math in straightforward ways or in rote memorization ways. They teach the same concept, in MANY different ways. Visually, verbally, with manipulatives, etc. and they teach these things, in a way that the child has to "derive" the answer... using deductive and inductive reasoning.
It is not just memorizing.
So, maybe that is what is throwing your son off about it all.
For example: When having tutors, they teach things in their own method. To the child. But, if the child is not learning math/reading in the same manner that it is taught at school... the child still may not, be able to grasp or do it... like in school. Because in school per the curriculum... the child has to be able to analyze/construct and work out the math or reading problems, as it is taught, in school. So sometimes there is this learning conflict between the two methods.
ie: my daughter had a Math Tutor when younger. BUT, the Tutor was a Teacher as well, who knows the school's system and curriculum and methods in which the students are taught, at my daughter's school. So she taught my daughter, according to those methods. Not her own methods.