If your good enough then your skills, mentality, toughness etc is all good enough. Especially for a half. Either you belong in FG from the word go or your doomed to be a Jeff Robson quality half.
Props and backs can grow into a career, i'd argue if a half hasn't proven himself in FG by 21/22 then he's a long shot to ever do it.
If he gets through the trials, start Henry round 1. He's not a soccer player converting to RL, he's played league for years and is a big boy. He'll adjust to the pace and intensity. He might not kill it, he might even have a shocker but he is a confident kid and i'm confident he'll be a success as long as we don't fart arse him around between starting, bench and lower grades.