When do I tell him. . .

I suffer from memory loss due to the fact that I once was addicted to vicodin. I haven't had any withdrawal and been doing great for 6 years now. I tend to forget words and have a hard time explaining things I just read or heard. I am currently dating . Been dating a guy for almost 2months. Sometimes it's hard to talk and converse due to the fact that I have hard time putting sentences together. Or remembering events. When is it a good time to tell him the reason why I have a bad memory? And that it's due to me past addictions to Vicodin?