+1 on just tweaking the next batch (I have had two batches come out with a significantly lower ABV than I wanted, but oh well, that just means I need to make more beer!
One of them is actually quite pleasant, the other one was wrecked -- though still drinkable, just not very good -- due to completely unrelated factors)
That said, if you want to add booze to it, I think it would depend a lot on the style. I would not do it with pretty much any light-colored or crisp-tasting beer, but if it's a porter, for example, a little bourbon or scotch or something might actually be pretty tasty, I would imagine.
Plus that, people here haven't heard of a boilermaker?
Your best bet is to not do anything with it, and just make the beer you want next time. If you are bound and determined, you could ferment it more by adding some sugar or some dry malt extract or something, or depending on the style you could try adding alcohol (though as others said, I would do it in small batches or even at serving time, in case it turns out to be nasty).