I'm 49 years old and, drinking or not, I have a hard time getting an erection. It's just a fact of life that guys run out of testosterone. I find it more desirable than being guided by my loins and being led to distraction by constant thoughts of sex. I welcome this change since I'm single and have no one that I have to perform for.
It would be nice to have an erection on demand, but oh well. I could try any number of supplements on the market, but it's not really a big deal for me. I've had enough orgasms for one lifetime.
When you stop drinking, all kinds of things have to adjust in your body and mind. 3 years is a long time though. Could be psychological rather than physical with regards to your boyfriend, but who knows? Couples therapy couldn't hurt. Then again, it could be entirely physical. At least the viagra seems to be working when needed.