Tis always wise to look before you leap...
As someone who spent 10 years in commercial diving, may I respectfully suggest you do alot of reading & ask alot of questions prior to plunking down major coin to learn the trade.
"Underwater welding" seems to be the idea the lay public has of what deepsea divers do when in fact, it is only a small part of the picture. Welding u/w can be done in the "wet" ( sticking metal together in water ) or in the "dry" ( subsea habitats seal around the job, the water is evacuated & the welders weld in the dry ). "Dry" welders are called "hyperbaric welders" and are highly trained/experienced topside welders who learn to dive & ply their trade underwater. If your son is serious about welding, suggest he aquire as many certs. as he can, spend several years honing his craft surface welding, then go to dive school.
There is "inland" & "offshore" comm. diving. The inland divers are heavy into u/w construction & work on dams, bridges, pipelines etc., etc., in harbours, rivers & lakes. Low to no visibility, strong currents, working below the mudline, polluted waters are all aspects of the inland divers' life. The pay varies: union jobsites pay substantially more...if you can get into the union; non-union pay rates vary from site to site ( see the link for more details on remuneration ). "Offshore" generally refers to oilfield diving...drilling, production/delivery & maintenance. It is a big world. Big ships, big barges, rigs, platforms, diving systems...the scope & scale will knock your socks off.
As a rookie oilfield "tender" ( earning the right to be called a "diver" is about a 3 year apprenticeship ) you work hard, listen hard & say next to nothing. Go offshore with an attitude & you'll be on the next boat for the beach. Dive school will give you the basics, the real education is out in the field. You need a thick skin & a sense of humour to survive out there. Gulf of Mexico divers have a long-standing tradition of treating green tenders as so much cattle fodder until they prove their worth...then they will slowly encircle you into the fold of the brotherhood.
Check out these links & if you have more questions, I'd be happy to try & answer them.
http://diveweb.com/uw/archives/arch/janfeb00.02shtml
http://offshorediver.com
Regards,
DeepSeaDan