jen, being an alcoholic in recovery i feel i can safely say he's using. for example-the meth test. he would've had to take enormous numbers of OTC to even be part of the possible equation. i don't even know if the OTC would've caused it to show up as meth. anyway either. reason-meth is made with some of the OTC stuff in it. it's not just the chemical in OTC. and the meth test would've shown high quantities vs. normal. that's why the test is designed to filter out users of meth not legal drugs.
secondly you've already found some meth in his room. a drug addict or alcoholic will PROTECT useage at ALL costs. whoever brings in something from their car and it isn't that person's? if a drug addict had been in his car u can know they would've taken their drugs with them!! not leave it in another person's car.
i'm so sorry you have to deal with this experience with someone u care about. his life will go downhill from here. the job thing is an example. if he keeps using his life will continue to spiral out of control. anything you question or say will NOT make him stop. only HE can make himself stop.
if it were me i'd walk away. his using will become your problems too. his keeping drugs in your house makes you an accessory if police get involved.
i'd suggest your posting this thread in the "addictions forum", too. ex-drug addicts will respond. note these are my opinions but i don't think i'm wrong. i'm glad u posted. i know you are hurting. we're here for u, jen. please keep us posted.