objective versus subjective on IEP

I have been doing a lot of research on IEP’s. The one thing that I need help with now is advice on what would be an objective way of measuring my son’s progress each year. So far, it has been measured by using “teacher observations and test results”. The tests are used are always just the classroom work. I believe from what I have read this is considered to be subjective measuring. No two teachers can observe a student in the exact same way. The tests prepared by the teachers will differ even for the same subjects. So what should be used to measure his progress objectively. (I understand this to mean that no matter who you give the test to, the test takers will always fall into a certain “category” with a certain “score”. Am I correct?)

My son’s problems are mostly with writing, math, and organizational skills. He is just a little below grade level in reading.

Thanks for any help.