How do I help him stop? I know he wants to because when we start to talk about it i can tell.
If the subject comes up again, you can tell him to grow up and start being reponsible for himself.
If he wants to know how STDs work and soemtiems work together (ie, those with certain STDs are more prone to also getting others STDs including HIV) there's a fair bit here: http://www.smart2ask.org/STD.htm
He's a lovely boy, but seriously...just get a fuck buddy :/ I said that to him and he was like "no..cos they always fall in love with me."