I echo the point others have made about what your son actually wants. We've had apprentices and trainees through who were there because their parents knew someone who works for the company, and had managed to wangle him a job. This isn't meant to be a generalisation, but in the instances I refer to, they all ended with the kid pursuing other avenues when they found out that what their parents wanted wasn't what they wanted.
I know we all want what's best for our kids. I also know you can't put an old head on young shoulders too. So I would get him to do the legwork if he's interested. Give him what help, advice and encouragement you can to pursue what he wants to to, but it needs to be him driving this. Building trades are a rough old place. He'll need a bit of "get up and go" to be able to cope.
I would say that if possible get into into the trade as a mate. It may mean not getting paid on college days, but making necessary sacrifices is worth it in the long run.