The way this guy is treating you is unacceptable. You told him you were pregnant and he took off for 3 days? That's ridiculous! If this has been happening over and over, and he still refuses to change, then tell him to either go to counseling with you or get lost.
I'm not sure what the laws are as far as a 27 yr. old getting a 17 yr. old pregnant. Could he get in trouble in your state? I will have to look up that law, or maybe someone on here will know. Are you emancipated?
Assuming for now that he can't get in trouble in your state, then sit him down and have an honest talk with him. Pick a good time when it's just the two of you and he is sober. Explain to him that since you both are about to be parents, he needs to grow up and act like a father. His days of drinking and staying out all night are over. Tell him if he wants to be in a relationship with you, he will have to show you he has changed, not just tell you. Ask him to go to couples counseling with you. Even if you don't stay together, it might be good for you since you will be raising a child together. Tell him that if he continues to go out and drink and show a total disregard for you, that the relationship is over.
Stick to your guns. If you tell him to change or else, then you have to be prepared to enforce the 'or else'. If he doesn't change, give him a reasonable date to get out of your house. Don't ever stay with someone who treats you badly just because you have their child. In a perfect world, all parents would live together and everyone would have the classic 1950's sort of idyllic childhood. But this is the real world, and it doesn't always work out like that. It's better for your child to have one good parent, than be around an alcoholic with no regard for his family.
Now, I'm not necessarily telling you to break up. Although I have to admit I think a guy his age shouldn't really be with someone who isn't 18 yet, no offence to you of course. He should have at least been smart enough to use protection. But anyway, the thing is that you can't change other people. I tried for several years with a guy that I cared for very much. But, the bottom line was that as much as I loved him, I couldn't change him. I had to let the relationship go, and today I have a great fiancée.
So don't sell yourself short. If he doesn't want to change and quit drinking so much, then you may have to tell him to leave. Maybe it will be the 'wake-up call' he needs to change. Maybe he will still do the same things. But either way, you won't have to deal with what you are dealing with now. It will only get worse when the baby comes. You need to either settle it now or be done with him before your child is here.
I know it can be so scary to be pregnant and single. But, if you guys do break up, there are lots of organizations that can help you out. There is help for housing, groceries, medical care, etc. I noticed on your profile that you live in Missouri. If you ever need help for you or your baby, contact the Missouri Dept. of Social Services. Here is their website: MO HealthNet for Pregnant Women and Newborns
If you ever need to talk or just blow off steam, I'm on here a lot, so just send me a message. I hope that things work out for you. Just try not to stress as much as you can. Good luck!