About the MAP scores, unless your DS took the 6+ version, the numbers may not be accurate. Assuming that the scores were either from the 6+ version or otherwise accurate, the numbers may still be too low for high school unless your DS is stucked in a district without students who can hold their own nationally (top 90th percentile or so). As a comparison to the over-inclusive "pseudo-GT" classes for DS/DD's 8th grade cohort, those Math and Reading scores would have placed your DS among the low-performers.
The key is figuring out the composition of students at your DS's school. Another poster mentioned that their school has a "middle school Algebra I" class, which is watered down and required repeating with a "high school Algebra I" class. In our district, it is the complete opposite! The Algebra I class in 7th grade has the bright kids while the Algebra I class in 9th grade has the remedial kids - this is objectively verified by superior performance on a national Algebra I test near the end of the school year.
I am not certain about the 50% threshold. Is it 50th percentile for targeted grade or 50% correct on end-of-course assessment. The former would be too low a threshold but the latter may be reasonable. The consideration is whether you want your DS to be in the middle of the pack or among the top performing students.
I highly recommend you steer your DS to the Art of Problem Solving website. Try their readiness and mastery tests for Algebra 1 and Algebra 2. There are also tests for Algebra 3. Do note that a solid Algebra I course at a B&M school actually covers part of AoPS' Algebra 2 course while a solid Algebra II course at a B&M school would cover parts of AoPS' Algebra 3. However, AoPS is more in-depth and challenging. There are also the free online Alcumus problems categorized by Algebra topics. You mentioned that your DS is completing Algebra II so if your DS quickly hits blue (mastery) on the relevant Algebra topics on Alcumus, that can also reassure you as to his readiness for Geometry.
