My son has to take a hair follicle in mid January he was in a rehab for 7 months and got out in sept. he relapsed but did not smoke much maybe a blunt and couple joints from what I understand but nothing since beginning of December will it show up:(

It's very possible it could. Actually really possible.

Please check this site out...

Erowid Drug Testing Vaults : The Basics

A hair follicle test for drugs measures the drug molecules and there metabolites that are produced after the drug has been processed by your body. Trace amounts of the drugs metabolites are inserted into hair follicles by the blood stream, The drug screen test usually covers basically around 90 days.

Yes it is very likely, and not much is still smoking it, and every time the perosn says a little, you can bet it was a lot more.