Should I just give up....?

I have been with my boyfriend for 4 years, and it just come to my attention that he is bipolar(untreated). He drinks, smokes marijuana and drinks promethazine cough syrup to get "high". When he is "okay" our relationship is wonderful. He can be the most loving person you can ever want to be in a relationshp with, but then there is the flip side and it's like Dr. Jekyll and Mr. Hyde...he flips out to the point where I am very afraid of him. We have a 2 year old son together and I so much want him to be a part of his life, but I'm afraid. He is 29 years old. Is it too late for him to get help? Should I give up on him and raise my son on my own? He has a 6 year old daughter and she shows signs of ADD/ADHD and often acts out, and I don't want my son subjected to this type of life. I apologize for the length of this question, but I am at my wits end right now. Any body that has experienced this from either end of the stick, please share your experiences/outcome from the situation!