He's 6. He just wanted attention and was probably just imitating somebody else he saw.
Besides, what would a demon (or spirit in general) gain from making somebody roll around on the floor like an idiot?
Well he wasn't rolling around on the floor but to answer your question, a demons sole purpose is to gain the control of a persons mind first, then body. They seek to live inside unguarded souls to defy the word of God and ultimately, lead that person to an untimely demise. So gaining control of a kid would mean that very thing.