My son was involved in a shooting. A man pulled a gun on him and he shot him in self defense. This man's?

friend was present and took the gun from the scene and now my son is being charged with 1st degree. From the 1st he stated that the man had a gun, they bagged the mans hands at the scene but according to the prosecutor, no test was done. THis is a small town and the parents of this man are well known here. My question, is there anything that can be done to show that the state did ot do the test because they work with the prosecution. My son has been told that this could be a major issue that should be brought up before his trial and possibly get it thrown out. Please no stupid answers, this is very, very important and will affect my sons life. thank you so much