I had done a phaturnity test done when my son was about 6 months,and turned out that father on the birtcertificate and who's last name he has is not the biological father. My son now is now 22 months. Did I wait to long? Will I have to do a test again?

The first test should be OK. You have to get ahold of the stste records office, then they will have ither a form or may need a written request. It verys with each state.