moved to US public high school -not accepting IGCSE grades from Cambridge International Examinations

We moved last summer to US from India. My son did his final IGCSE and got all straight As but the high school which initially accepted the final IGCSE grades now used his internal non standard grades from his old high school in india during his 9th/10th grade and left out the final IGCSE grades. My son's GPA has been dented from being above 4 something to 3.9 something. Has anyone known of such a situation or know of any schools in US that accept final IGCSE grades? I have even shown the letter from Cambridge International Examinations that suggests to take their grades and that they are internationally evaluated in the UK and are an accurate indicator of performance than the internal grades that different schools evaluate differently but they are adamant in accepting it.