So as per topic, what should i do? He is standard 5 iinm. I dont care if he watches straight porn since i think most of us did the same at that age 

Found out by checking the browser history since he always use the computer in my room whenever im not around to play games and watch youtube. But one particular day he locked the door whereas normally he wont even close the door properly. Since then i always check the history and yeah he did watch it on other days as well.

I want to tell my sister about it so that something can be done to correct him before its too late, but at the same time i dont want my sister and my bro in law to be very disappointed that their son is gay. And also im afraid he will do something with my kids if i expose him, although i can ask my sister to say she found it out herself by checking the browser history

Another possibility would be he was simply curious on how to masturbate 

His google search was always “boy cum”