We are not rational
Ever see Freakonomics? We humans are great at ignoring logic and evidence.
So trying to convince your BF to stop may actually work against you. He likely knows the reasons he should quit (the things you say to convince him) but I assume he does not yet want to quit. I'd suggest letting him know something like he'll decide if and when he wants to quit, & he'll decide why. Give him nothing 2 resist.