It's not as complex as him stopping to think about choosing drugs or choosing you. It's an impulsive act, based on urges. Just like if you are on a diet and you notice that someone has left out a candy bar. You might say just this once will be ok, I've had a bad day and before you know it, you've ate the whole thing and feel terrible. Trouble is, the next day when that sugar high isn't so high anymore, another candy bar pops in your head and now, after the initial screw up, do you do a little more thinking: you know that the candy bar tastes really good. but this candy bar also makes you feel really happy for a few hours and you've been feeling crappy. In the back of your mind, you know that you shouldn't, but you got clean before, this is just twice so it won't be as hard this time. You would give anything just to feel better for a few hours. And there you have it. It's tough to choose to walk away, sometimes. His relationship probably didn't enter into the equation, it was most likely a false belief that it was only this one more time and you wouldn't ever have to know since he planned on just one more time. Nobody sets out to ruin their own life but drugs are more powerful than we bargain for. Best thing to do is separate from this guy and let him see the consequences more clearly. I hope he gets better for his Son. Good luck.