My son is charged with 23 counts of possesion of child porn.(did not ask for any pictures)

He was on a dating app called meet me .com.He was texting with a woman and she sent him a picture of herself and her stepdaughter topless.He didn't ask for pictures of any kind and didn't know the picture he had to download to open was going to have a minor in picture.He deleted picture but was arrested anyway.The woman that sent the picture was evading police for 7 months but finally arrested.He has been sitting in prison for almost a year but has not gone to trial yet or has he seen what evidence they have to hold him.How can they have 23 counts.