Based on some studies, it proved that hair drug testing significantly detects drug usage more than the urine test. It only takes approximately 4-5 days from the time of drug use for the affected hair to grow above the scalp, thus hair test done after few days of usage will usually show up positive for the drug use. Hair test is usually very sensitive and can be even used to identify the deter mine low (occasional), medium (weekend, daily) or high (constant) usage. However, for the detection of marijuana it is said to be less sensitive than the other drugs in identifying low level drug users
In the case of your son, it is quite strange to have been tested positive in urine and did not show up in the hair test. There could be possibly a significantly low levels of marijuana metabolites present in the hair not enough to yield positive result. A Gas Chromatography Mass Spectrometry (GC/MS) should be done to verify his marijuana usage.