If your brother is still showing any of these signs, you should probably take him to a doctor.
* confusion or feeling dazed
* clumsiness
* slurred speech
* nausea or vomiting
* headache
* balance problems or dizziness
* blurred vision
* sensitivity to light
* sensitivity to noise
* sluggishness
* ringing in ears
* behavior or personality changes
* concentration difficulties
* memory loss
But from what you said happened, if your brother did in fact get a concussion, it was probably no more than a Grade 1 Concussion, which means those symptoms only last for 15 minutes or so. But since you said it happened four days ago, and your brother is doing fine other than being pissed off at your cousin, then I wouldn't worry.
Well, my brother did have a headache for over 2-3 days aswell as a little stammer or so more than usual.
Should I still take him to the doctor? :S *Never was good with the Health Department. :\*