Lactose is the sugar from milk that is added to beers. Lactose is just a non fermentable sugar, I don't find it super sweet. Maybe try a oatmeal stout with some maltodexterin and vanilla. You need something that wont ferment out. Things like sugar, honey, etc will completly ferment out and give the beer a dryer finish. There are things like splenda that people will backsweeten with, but it is usually for fruit beers, ciders or wines.