There are a few great ways to do this. The best, I've found, is to bake your cake and let it cool. While it cools, make a syrup of the alcohol you want to use. You can do a quick search for this online, but it's typically 1:1 ratio of booze:granulated sugar. You put your desired booze (bourbon goes well with chocolate, triple sec with citrus or pound cakes, etc) into a saucepan with sugar over medium high heat and whisk until the sugar dissolves, then let boil until reduced by half. When it's reduced down, it's a much sweeter version of your favorite liquor or liqueur.
Then take your cake rounds that have cooled and level them, exposing all of the little pockets of air (or if they don't need leveling, prick them with a fork repeatedly to create little holes). Take a pastry brush and brush the syrup over the exposed tops of the cake, allowing it to seep into the cake. Be careful handling these cakes, however, as you have just introduced a lot more moisture and they can crumble apart more easily.
To infuse your frosting with booze, simply add a couple of teaspoons (to taste) to your frosting at the time you would normally add your vanilla or other extract. You can do this with whipped cream, too. Or, you can make a liquor-infused ganache by adding a few teaspoons of booze after you have poured the hot cream over the chopped chocolate, then continue to add until you have the strength you're looking for.
I made a dark chocolate whiskey cake once with a dark chocolate whiskey ganache once that was superb. Also great was a citrus cake I topped with Cointreau whipped cream. So many options! Also, if beer is more his thing, there are a lot of great recipes for chocolate stout cakes (made with Guinness or other stout beers) that go in the batter.