Guys, I really need help.

My brother was in a 10th lobby and killed about 10 people and got minus (-) xp and has
-10888159xp. He is level 1 and cannot search for a game because everytime he does it says he is not level 1 so he cannot. Also, whenever he gets invited he can join a game online and kill people. But when he is invited into another 10th lobby and tries to kill someone for xp, he gets disconnected straight away.

If someone could just help me resolve this, or even help fix it, I would be grateful and provide payment via PayPal if the method works.

Add De CoDfAtHeR (My Gamertag)

Thanks!!