It depends on his interests, skill set, and where he's like to live as well as lifestyle.
HVAC combine with plumbing (heating and cooling) can be lucrative and a stable/steady job in any market if he is mechanical minded.
Pipefitting/steamfitting, gas fitting, as well as steel fitting if he is mechanical minded and is interested in working in the oil & gas industry, and perhaps ship building.
Electrical is use in every industry, however it is considered as a sub trade and depends greatly on other trades.
Heavy duty mechanic is a support trade that also use at shipyards and in the oil/gas industry.
Instrumentation mechanic if he is mechanical, electrical, and computer/problem solver minded person. This trade is well pay and is well respected in aviation, ship repair, and oil/gas plants.
IMHO, one can't go wrong with any of the above trade.
I have the following degrees and certs: industrial design, computer science, plumbing, gas, HVAC, pipefitting/steamfitting. And, there are oil/gas industrial contracts that I can take in North America as well as Australia with salary that are 4X more than what I made in IT/industrial designing.