I would approach him, looking like someone from a really fucked up apocalyptic future. I would learn all the details I could about him and say to him.

"Hey (whatever his name is), you need to listen to me very carefully. I am from the future and I am here for you."

For proof I would start saying random things such as.. your Mother will win 500k on a scratch card 6 months from now, or your brother will get a pay rise in 2 years, whatever. Things that I can influence with my infinite wealth, I would make them happen on those exact dates.

He wouldn't believe me at first but I'd act very stressed and extremely concerned for the next part.

"I cannot tell you the specifics but you will know when the time is right. I have to tell you DO NOT DO IT. You lived a totally ordinary life until that point and then you sent humanity down a path it never recovers from. I am here illegally, I will probably be executed on my return, but if you have any good left in your heart and if you care at all about those you love and everyone on this Earth, please.. don't do it."

Then I would act suddenly very scared, as if something was coming for me and I would run away yelling "Remember what I said!"

At first he would dismiss this of course. Anyone would. It'd be a weird encounter sure but it wouldn't affect it. However as time went on my plan would slowly come to fruition.

Everything I said as proof, whatever it may be, I would make happen one by one. Say a relative winning a large amount of money through a lottery or whatever, just approach them and say there was a miscalculation with the machine that printed your card or whatever, make some bollocks up. There are loads of things you could easily use as proof that would make your time travel story seem true. You'd keep doing more and more of them to dispel any doubt from his mind that they were random.

Eventually he would become convinced you were real and this is where the life ruining starts to happen.

DO NOT DO IT.