I would ask why? And move on.

I would tell him that there are these "special summer camps" that will fix him..

“Get Out!”
Just kidding of course.

I have that fear every day. I have two step sons that have been with me since they were babies, and their bio father is religious. So alternate views are inevitable in my house.
I have always told them, I will love you no matter what, but don’t expect me not to challenge you with whatever you believe. Even if it’s the same as I believe.
I tell them it’s more important to know HOW to think, rather than WHAT to think.
But that’s tough to watch. I know some don’t feel as strong as I do about it, but to me it’s almost the equivalent of watching a loved one struggle with drug addiction.
Good luck to you sir.

Who is she?

Point him to people whose lives have been ruined by it?‍. Remind him he only gets one life, and encourage him not to waste it on insanity...?

'Holy Shit!'
Sadly, I'm not kidding.
When my son was in pre-school the kids next door told him Jesus was in the clouds, looking down at them every day...serious bullshit. I took him outside and we looked at the clouds and we didn't see anyone up there.
Hope you go at this hard (NOT hard on him but don't be wishy-washy, tell him what you want for him, what you believe and why this concerns you assuming it does). Regularly engage him: find out why it's appealling, who's the influence? What is happening in his life that he's connecting in that way? I really think that if a kid has someone to help them through, they don't buy into bullshit and evangelism IS bullshit. Evangelism is cultish. The parent has to be dilligent, it may not clear up with one discussion and he might go this route anyway. Hope you give us some followup, Nevadarich.

Swat them with a rolled up newspaper, and tell them, "NO! Bad teenager! BAD!!"?

I would sit him down and ask him many probing questions about what he understands about the ideology he is buy in to. The choice is his. Your job is make sure that he is making an informed choice.

I would say, "Boy, what the hell is wrong with you?"

You say, "What dat chit uon, man?"

If you are truly an agnostic, don't you at least have to entertain the thought that they might not be wrong. After all, we don't know.

Following or not following any religion is choice everyone should be allowed to make for him or herself. It's his choice to make. It might be permanent or temporary. Be there for him when he needs you regardless of whatever decision he makes on this subject. (just my 2 cents)

Stone him to death--it's Biblical.

Good luck with that. Do what your heart says to do.

How old? Old enough to make decisions? Mother involved? Lot of variables.

What you resist persists

I'd take something like the Socratic approach. Ask him questions concerning the things make evangelism compelling. After a rich and honest discussion, one of us will be convinced.

Think about what most people do when you say do not do that. They do it anyway. The best is to let him learn for himself. You can gently guide him and help him find truth with the facts.

If the money's right....

I'd tell him it's completely unacceptable and you raised him to be smarter than that.

“Hold on a goddamn minute.” ?

It's up to them but they knew my beliefs and havnt.

If you follow any religion, on a global scale more people will think that you are bat shit crazy than not. It's not a great argument though as if you were to place the word don't before follow, it's still true.

I couldn't answer that as I have no children, but if I did and they started believing in imaginary people, a nice long chat would be in due course.

Read more books and study Roman history