My grandmother uses undulationed voices a lot. He could be copying you. You must be a very nice person. But it sounds like he is bored. The voices are what you do if you have ADD but if he is doing it constantly then I'd assume he is bored. Not just bored but probably experiencing the brainwaves that come from not taking the dextroamphetamine and that is what he does to avoid that experience or bring himself back from it. What kind of medication was he on?