What do you mean "put him up to it"? You think there are CIA agents hanging around going "psst, hey kid, wanna shoot the president?"
It's the heist-movie conundrum. There are way too many parts that have to go together. Somehow you have to get all cops to ignore the guy climbing on the roof. You have to have the guy actually go through with it and not go straight to CNN with his confession...
Just get Trump at the golf course, or at the Mar-a-Lago buffet. Trying to off a president at a rally is some Columbo bullshit. That might be the single hardest time to do it.