Your son got 37 of these items correct. The average student in the norm group got 44 of the items correct. These tests are constructed in such a way that students are expected to miss several items. Though your son is at the 35th percentile, he missed only seven items more than those at the national average of 50. In the big picture, seven items is really not very many on a 78 item test.
Percentiles have a way of exaggerating the difference in achievement among the various ranks.
You will want to speak with your son's teacher about his strengths and shortcomings in mathematics. Consider his math grades, and look over his homework. However, I would use the 35th percentile as only one indicator of his overall achievement, and possibly not a very significant one when several criteria are considered. Remember, percentiles compare students on a scale. It doesn't matter how much difference there is among the number of items they answered correctly.