Teen years are gonna suck for parents, I feel for you. Your son is going through the same stuff all teens go through in those painful teen years. Sneaking out of the house with your girlfriend, sneaking into her house, underage drinking with your best friend, and experimenting with marijuana are normal for a teenager. You shouldn't enable the kid by going easy on him, teens are pushing boundaries and they need to know mom/dad will be there to lay down the law when they get caught. Do not enable them, they need to know you care enough about them to grab them by the scruff of their neck and drag them kicking and screaming at 0200 from a party to take them home. If you don't place boundaries they will go nuts. Look on the bright side 2 more years until they are legal adults. Tell them to join the military, trust me it squares away many rebels.
Remember your not the first parent to have this issue and you wont be the last. There is a light at the end of the tunnel. They will appreciate what you do for them, not now but later down the line. I am talking from experience, mom and dad were right.
Just don't tell them I said that
BTW you raised him well, think about how many teens would rather lie than tell the truth about their activities. The boy gets brownie points cause he didn't lie to you. So things aren't all that bad.