Even back then they could've matched the knife wound to the knife itself (for the most part).  I do think that the test he suggested would indeed have been thrown out for the reasons specified (but I'm not a lawyer, nor do I play one on TV or the radio).  I did like the part of the father - he kept saying what his son said MUST be true, and yet he didn't believe him enough for the test to be done.  My impression of it was that the son was either really stupid (allowing the test when he knew he was guilty), or really innocent (knowing full well he was innocent).  It seemed like more of the latter than the former to me.