How do i explain to my son about size?

hi, my son came to me today really worried about his , he asked me a number of questions that i found very hard to answer. he told me that he is worried about the size his ans testicles and also how he finds it hard to masturbate successfully and how to do it . this was a shock to me because i generally thought that they are tought this at school, but obviously not. i told him we wud talk talk about it tonight, wat do i tell him cause i dont know the answers myself?