Unhappy, this is gonna sound a little harsh, but...
1--Is his life insurance paid up? Mixing alcohol and opiates is playing Russian roulette...eventually, he's gonna mix too much of each and it'll be game over. Just google Karen Ann Quinlan.
2--It's one thing to disrespect your spouse, but he has TWO young children? That's WAAAAAY beyond selfish, that's not caring that you have children.
3--Run, don't walk, to a lawyer. Even if you kicked him out of the house, he is still obligated to care for your children, even if only financially. Separation or divorce, doesn't matter, but talk to an attorney, and NOW!
4--Don't get involved with his workplace. If you say the wrong thing, or they interpret it incorrectly, it could come back to bite you.
5--Terminate the marriage. Yes it sucks, but you must look at it that HE screwed up, not you. HE has chosen alcohol and drugs (whatever they are) over you and your children. You mentioned that you've taken the kids out to do things, while leaving him at home; good for you, and good for your children! It sure sounds like it's time to move on with YOUR lives (you and the kids).
6--Again, check on your legal options NOW! Do not wait until something tragic happens.
Your last post kinda says it all...it's very apparent that he doesn't care about you or his own flesh and blood children anymore. Until he is willing to do something about his drinking and drug use, you need to get yourselves out of that situation. Don't lie to the kids, but don't give all the details either; just tell them that daddy has a problem that he has to take some time to get better. This way, you don't look bad in your kid's eyes.
Also, should you wisely choose to separate, make sure that the agreement/lawyers/court make it clear to him that if he shows up drunk or stoned to see the kids, they don't go with him. I mention this because a good friend of my wife had this situation arise with her drunken ex-husband THREE times in the first year after they separated, and he lost his priviledges for 4 months, until he sobered up and completed a court-mandated rehab program.
Good luck to you, and keep us posted.