It would be a good thing to let your parents know about it… I assume you may not be too comfortable discussing it openly with your brother but you can always show him the beauty of a chaste life… talking to him about saints who practiced purity like St Maria Gorretti, St Dominic Savio, St Aloysius, etc may prove to be of some help… to him stories of younger saints may be more appealing to him as he can relate more towards their age.
Being a 10 year old kid he surely is in a stage where he can be shaped to be good or bad and can easily absorb influences be it good or bad… also stories about the visions of St John Bosco with regards to boys going to hell because of the sins of the flesh and the visions of the seers of Fatima are just some examples that can be helpful to deter a person from indulging in such activity.