Not sure where in construction he was, but I've worked framing crews before, and the boss would hire anybody (not disabled) who said they were willing to work hard. It isn't hard to teach someone the basic tasks and most people could catch on in just an hour. Most people though realized it was physically hard work and quit in a week (even though this should be obvious).

Plumbers and electricians have a good start from nothing apprentice program. They are sometimes hard to get into, but I know a few people who have gotten in over the years.

If you are training people and they aren’t staying, isn’t the implication that you aren’t paying them enough? Those union apprenticeships are often hard to come by because 1) they pay quite well compared to other viable options and 2) people don't want to leave because of #1. If the job is demanding, the pay should be commensurate. The exception is if it’s rewarding on some other axis (e.g., socially).