My 22 yo son is a recovering alcoholic. I had a similar experience of cleaning and finding an incredible amount of hidden bottles. I had never seen my son drunk at the time - he managed to hide it and do most of his drinking after we went to bed.
My past year has been a series of hospitalization/detox/relapse/hospitalization/detox/30 day rehab/6 months sober living/relapse/21 day rehab- so far, sober 60+ days. I write that to show that it can be quite a roller coaster ride as our loved ones attempt sobriety. Some get sober and stay sober the first time- although they are in a small minority. More do what my son has done- periods of sobriety followed by a relapse.
Your original question about how to trust again- its very hard. I understand for you its especially difficult because you can't tell when he's drinking and fear you will be fooled again. I agree with PP and read and educate yourself on addiction and codependency. You don't have to make an immediate decision- but, you may decide its a ride you want to get off of. I think if I wasn't a parent, I would have stopped putting up with my DS a while ago.