Forget machinist, those jobs can be sent over seas. The building trades cannot be sent outside the country.
I was Exec VP of a large commercial MEP contractor and had my Master level HVAC, Plumbing & Gas Fitter cards. I would not recommend being a welder unless it is combined with plumbing and pipe fittings so when his eyes are shot, somewhere around 50 to 55 years old he can still work the pipes.
Being an HVAC Tech is a good career if he gets into commercial work, and that requires considerably more training than the average HVAC Tech wants to do. Most go into a certificate program in a technical school get a job with a residential/small commercial operation which will not pay as well as a tech trained to work on large commercial equipment. If he goes the HVAC route tell him to continue his education beyond just a HVAC Certificate from a Tech School or Community College. Get him to get on with a commercial contractor, a manufacturer such as Trade, Carrier, York, etc. and to take all the classes they will let him have.
If HVAC is not his cup of tea I would highly recommended the electrical/electrician trade. Typically an electrical installation is not a strenuous physically as a plumbing/pipe fitting install is, although there are exceptions. This means his career can be possibly be a little more productive late in life over the guys who are plumbers/fitters, just due to the stress and strain on the body. Todays electricians get to do some really cool installations with the computerized lighting controls, security systems and computer networks that modern building have today.
If my grandsons want to be in the trades I would recommend Commercial HVAC or Commercial Electrical as career paths.
