Good material for stand up comedy?

My nephew, 14, is fab at stand up comedy and wants to try and get some good material together. The trouble is a lot of what is available is not suitable for his age as it is either very political or dirty. Although really funny, I wondered if anyone could suggest great stand up comedians that are still funny without resorting to 'below the belt' jokes... observational kind of stuff without crudeness. Any tips of where to look? Thanks