Begin with a math placement test that your student can take in the privacy of your home. Don't tell your child that he is taking a placement test. There is no need to add unnecessary pressure. If he struggles with the first few problems, go down a grade level. Successfully completing a lower grade test can bolster confidence and help a child remember basic principles.
Much of a reading placement test can be administered verbally. Again, find a quiet place in your home for the initial test. Reading is significantly frustrating for some children, and a quiet place where your child feels most comfortable is important. Take frequent breaks.
Once you have determined the approximate level for your child in these important subjects, you'll be better prepared to speak with his school. Make an appointment to speak with the guidance counselor. Be sure to bring the tests he's taken with you to the meeting. The school will likely require retesting, but your tests will help determine where to start.