In law they are indecent photos of children, hence he is breaking the law.
As said, many parents have "innocent" photos of children re bathtime etc. However, even these photos can be class as child porn if the intention of the observer is to make them sexual. Of course there has to be a degree of nudity or meaning, hence photos of kids on a beach or playground, although disturbing if viewed as porn, there is no offence.
"Preteen", the actual law states that 18 (Eighteen) is the age of models in porn (the only exception being if married). So in the UK a man can legally have sex with a 16 year old but he may not sexually photograph her, unless married.
Report him. You can do it anonymously.