Just seems rebelious towards his mom. He is not out at all hours. Stays in his room with Xbox and computer. Very intelligent kid. He is White. Russian and peruvian.
His behavior is completely normal. He is at an awkward stage of being a teenager and trying to be a grown up. A 15 year old boy thinks they know everything about anything and will challenge just about anything their parents may say.
Your girlfriend needs to clamp down and establish rules/regulations/boundaries. Also, her son needs to be aware of consequences if the rules are broken. It all boils down to effective parenting skills, not being afraid to discipline your own child, and always remaining the authoritative figure.
Additionally, staying is his room and playing computer games may not be the best choice of activity especially if he is playing the violent games like Call of Duty.