his body language when he answers?

But he would probably know it is me.

and would ask him, nicely and without gritting my teeth, to tell me about it--how he decides who might benefit, what the therapy consists of (even if you already know), and how the long-term results have been.

If this election has taught us anything, it's that going toe-to-toe with people like this is pointless. They don't care about facts. I think, especially since this is someone you care about and who cares about you, trying sincerely to understand his motivation and talking to him about your own doubts--not bolstered with a bunch of print-outs from the American Psychological Assn., but your own doubts--is a way to keep communication open and allow him to find a way to change his mind. The more you present him with facts from experts, the more nut-jobs he's going to quote to you, and the harder he's going to dig in.

You might, gently and lovingly, ask him if, say, shocks to his genitals when shown pictures of naked women would make him become homosexual. And then talk about it, without forcing him to make a conclusion. Did he ever make a conscious decision about which sex he would fall in love/lust with? And not follow up with a triumphant "See? So why should it work for other people?"

If, on the other hand, he has stopped doing it, bake him some cookies.

I don't know what avoidance therapy could consist of, though, unless it somehow elicits a painful reaction when exposed to the thing you want the patient to avoid. I guess rubbing dog shit in the patient's hair while showing pictures of handsome gay men would work, too, but I'm not the expert.

will be saved. So if a gay person does it why do they tell them they are going to hell.

Because don't tell me everyone that prays that prayer is completely pure. There are a whole lot of sins talked about in the bible. One of them we here in the US celebrate. The sin of gluttony on Thanksgiving. (And all through Christmas too)