What Causes A Fluid Filled Bump On The Back Of The Neck?

Hi, may I answer your health queries right now ? Please type your query here... My boyfriend has a bump on the back of his neck. It's under the skin and you cannot see the bump, only feel it as you press on it and squeeze. When he squeezes on the bumped area pus like substance comes out. However, it is not really pus like. It's more like the inside core of a pimple; the white harder part inside the pimple. And this piece can be quite long that is squeezed out! Any thought? Should this get checked out? It's sitting on top/near his spine.