Josh Raymond is Finch's seven-year-old stepbrother. (Well, technically he may be his half-brother; Finch isn't sure how long his father was cheating on his mom before he moved out.)
Josh Raymond has everything that Finch never had as a kid, including a huge room and tons of toys. (Then again, he has to live with Finch's dad, who's an abusive jerk. Guess we can call it a draw.)
In any case, this kid mostly exists as a vehicle for Finch to talk about his big, bad feelings toward his father.