Speaking for all 16 year old boys with internet access. We watch porn, get over it. If you get your brother in trouble now you are bound to be embarrased pretty soon. Get off his metaphorical d*ck because nearly ALL guys watch it. Why are you in his history to begin with? Don't tell your parents, he will resent you and hate you. Don't get more evidence, you already saw his history, what next? Are you going to sneak through his stuff or something? That is SO rude and disrespectful. Just tell him to delete his phone history (without mentioning what you found) and he might let you off with a warning.
Don't be that kind of sister that he grows up hating because she embarrases him for no good reason whatsoever. Leave him alone about it, and before you complain further - it is completely NORMAL for ALL guys to do it, your parents most likely still do it too, grow up litte kid.